Elimination , DTMF Problem & Modem / Fax Problem Tech Bulletin
While echo problems were few and
far between in the past, eliminating echo
has become a challenge
when installing today's VoIP phone systems.
way to eliminate echo on a VoIP phone system?
sure that all of the lines connected to the system
are digital, delivered on a T1 or
That's easier said than
done since there are so many analog phone lines out there. T1s are
sometimes more expensive for both the monthly service and the card to
connect the T1 to the phone system. T1s are usually more difficult to
get repaired, and if a single T1 goes out it could effect up to 24
lines - leaving the customer with no telephone service (even with a T1,
it's a good idea to keep a few POTS lines).
Digital lines probably won't stop the echo if
it was created on the far end of the call or at the phone company
itself. If you have a T1
you know the echo won't be coming from the interface between the
phone system and the phone lines. A T1 may help echo created on the
far end of the call, if the T1 provider runs all of the calls through
an echo canceller in their CO.
It's usually not
economical for a customer with less than 10 lines to get a T1, but
it may be necessary to eliminate echo if it can't be fixed any other
way. Some CLECs are offering fractional T1s with only six lines at a
reasonable price, but it might be expensive or impossible to get a
T1 card for a particular small phone system.
It's worth doing a little
troubleshooting to see if you can get rid of the echo. If you only
spend $20 or $50 a line to get rid of the echo once (instead of a monthly
cost), that's probably a lot
cheaper than going to a T1.
HERE if you already know why you get echo and just need to
find out how to fix it farther down
go to the most common causes of DTMF Fax / Modem problems
The echo I'm talking about in this
bulletin is from the connection between the phone system and an
analog phone line, or an analog station port and the phone system.
If you have echo on an intercom call between VoIP phones on the
system, or only when using a speakerphone or a particular type of
VoIP phone, you'll have to figure that out with the phone and/or
Don't confuse an Internet T1
with VoIP phone lines, with a Channelized Voice T1 or PRI. If the
customer gets VoIP lines from their connection to the Internet, that may fix the original echo problem
- but the overall quality of the call will suffer because the voice
packets will be routed over the Internet in a way that may result in
garbled voice and echo (there's no way to control the path of the
individual packets making up a conversation on the public Internet).
If a digital VoIP phone line is
delivered from a private network (so the voice packets don't
actually travel over the public Internet), the voice quality should
be great. There should be no echo caused by the VoIP phone
system itself if it's connected through a T1 or Ethernet connection.
In some cases, a LEC or CLEC brings a
T1 into the building but because the phone system doesn't have a T1
card (it only has analog trunk ports), a Channel Bank is used to
convert the T1 to up to 24 analog phone lines. Actually, they kind of
look like analog phones lines, but most channel banks don't do a
great job of emulating a real phone line.
If the channel bank is
connected to a VoIP phone system you can have echo or other
problems, just like
from a POTS line. Echo might actually be worse with a channel bank
since there are two 2 to 4 wire conversions using hybrid
transformers instead of one on a regular POTS line
(on a POTS line the conversion from the
4 wire digital CO to the two wires going to the premise is done at
Ten years ago a customer with a T1
down was given a high priority by most LECs and CLECs. Problems were
fixed within hours. Today, there are so many customers with T1s that
a T1 is now a commodity, with a service level to match. If it's
totally down it can usually be diagnosed and repaired
quickly, but because of a lack of training and equipment at the LECs
and CLECs, more esoteric or intermittent problems may be impossible
to fix (and there's a lot of finger pointing from all parties).
Fixing an echo problem by switching from POTS lines to a T1 may be
like jumping out of the frying pan into the fire, but you might not
have a choice.
One other thing that can cause echo
and other problems is connecting a VoIP phone system designed for foreign
(from other countries) POTS lines to US POTS lines (or vice versa). While a 2500 set will
probably be usable anywhere in the world, there are enough
differences in phone lines between the US and other countries that
there could be echo, volume, and ringing problems when you use a
phone system on the "wrong lines." If you hook up a Bulgarian phone system to a POTS line here in the US,
it might not be pretty.
The first thing to check for when you have
While DTMF digits don't work well on a phone line with echo or
impedance mismatch problems, which is the main topic of this
bulletin, the first thing to check when you have DTMF digit
detection problems on lines coming from a VoIP device are how the
DTMF digits are handled by that device (or a VoIP
device at the caller's end).
If you push a DTMF button on an analog phone connected to some
kind of VoIP box in your home, when you're calling a Voice Mail or
some kind of machine controlled by DTMF (like telephone banking),
unless you do something special with that tone the audio touch tone
will never make it (so it can be understood) to the other end.
That's because of both the compression used by VoIP which changes
the tone, and the fact that packets containing audio can arrive out
of order - or not at all.
VoIP is designed for voice. You don't have to be able to hear an
exact representation of the voice at the other end to be able to
understand what the other party is saying. Bandwidth costs money and
is always limited to some extent, and voice quality isn't very
important these days, so squeezing as much voice into as little
bandwidth as possible is "good." Even with burbles and distortion,
it's not a fun conversation but you can usually get the idea of
what the other person is saying.
If you play a touch tone, music, or even modem or fax tones on
one end, the tones or music will sound bad, and most won't be
recognizable to a machine (computer) trying to decode them. That was
a real problem with VoIP until a solution was found to be able to
transmit DTMF reliably (mainly for telephone banking and voice
The VoIP DTMF tone recognition solution is called RFC2833 (or the
newer RFC4733), a protocol used with VoIP which essentially
removes the audio touch tone from the far end,
replacing it with a data packet that represents
that digit, which is understood by the VoIP device on the other end
of the call, and is reproduced locally so the touch tone is perfect.
Well, almost perfect. The biggest problem is when the VoIP gizmo
replaces most of the DTMF digit, but not
all of it. That leads to a double digit being heard by the
voice mail or whatever, at least sometimes (an intermittent that's
really hard to troubleshoot!). When all of the original DTMF digit
isn't replaced, the voice mail might hear 227 when someone is
actually dialing 273. That ain't going to work!
Try looking for something about RFC2833 in programming on the
VoIP gizmo you're working with, or even on the person's VoIP gizmo
on the other end (a near impossibility with a casual caller).
Compare it to a known working gizmo to see what's different.
By the way, trying to send modem, fax or alarm data may be
possible at a slow enough speed with a VoIP device designed for data
(uses a different codec with less compression), but it's not
100%. Sometimes it works, sometimes it doesn't. For an alarm, you'd
probably want something very very close to 100%.
lines (or telephones) with VoIP phone systems
fixing echo was easy, anybody could do it!
The hardest cases of trouble are when
the echo is intermittent. That's usually because the phone company
uses echo cancellers on some phone calls, but not on others. Some
COs may be equipped with echo cancellers for long distance calls,
but not for local calls. You have no control over whether the CO
uses echo cancellers or not. If you have echo or DTMF problems only
from certain COs or cities, that may be the reason.
You have absolutely no control of
where the incoming calls come from, where outgoing calls go to, or whether incoming calls are originating on a
VoIP phone line or system. Some of the echo is coming from the other end, and
you won't be able to fix it.
The easiest echo to fix is:
Echo that you or the person at the other end of the call
always hears on a VoIP phone system when you're talking on
an analog line or trunk
- Echo that you or the person at the
other end of the call always hears on a regular phone system connected to
a VoIP phone line (adapter), and where you don't
hear the echo when you connect your butt-set directly on the line (with
the phone system disconnected).
You've got a 99% chance of getting rid
of those types of echo by correcting the impedance of the line using an
Echo Stopper™ Adjustable Line Impedance
If you can hear echo on a
2500 set or your butt-set right on the line all the time, it's probably
impossible for you to fix it by doing something yourself at the premise. That's
because echo you can fix is usually caused by the delay in packetizing
analog audio into IP packets, and there's no delay created by
a butt-set. It's possible to fix echo coming from a VoIP adapter
if you hear it on a butt-set by correcting the impedance, but less
VoIP phone systems have echo cancellers
built-in. Some work better than others. Even the best echo cancellers can't eliminate all echo, but if you correct an impedance
mismatch it probably won't swamp the echo canceller in the phone system.
Sometimes it takes a second or two for the built-in echo canceller
to figure out how to get rid of the echo - it's training
itself for that call. If you only hear echo for the first
second or two, you may be able to get rid of that echo by correcting
the impedance with an Echo Stopper™. By correcting the
impedance the system's built-in echo canceller doesn't have to do as
much work, so it may be able to train itself in a fraction of a
second, rather than a second or two.
There's no way to anticipate if you'll have echo
problems with POTS lines. When a phone system is connected to a
digital line like a T1, the interface is very exact - adhering to
technical specifications, and there's no analog to digital
conversion on the trunks (but there may be a TDM to packet
conversion in the system).
When you connect a POTS line to the
system, the specifications of that phone line are so loose that what
you get from either the LEC or CLEC is just a crapshoot. The
interface for a T1 line doesn't change depending on the distance
from the CO (and there's a single clock that keeps everything in sync), but
a POTS line looks a lot different depending on the distance from the
CO, the type of CO, and how the line is conditioned by the phone
company. Nobody can guess how well it will work until you get it
hooked up to a VoIP phone system.
Taking a demo system out to a
is a good idea if the lines are already in when you're proposing the
job. If there are analog stations or OPXs involved in the proposal,
make sure you go as far as you can in setting up the system like the
customer would be using it - including swinging the OPX over to an
analog port on the
demo system to make a few test calls. If you hear echo, try an
Echo Stopper™ to see if it will eliminate it.
If the OPX doesn't ring the far end
and/or it won't work at all, try our
Ring Voltage Booster™
Loop Current Booster™. The analog station ports
on most modern phone systems and VoIP gizmos have lower loop current
and lower ring voltage and talk battery than in the past. If the
volume is low on an OPX, try our
Long Line Volume Booster™ at each end.
POTS lines usually work OK on an old
2500 set or your butt-set, but it's a whole different thing hooking
it to a VoIP phone system. When you have a problem with a POTS line
and report it to the phone company, they make a single phone
call from their butt-set and if it sounds OK, they'll close the ticket NTF
(No Trouble Found). They don't
care if it doesn't work with a particular phone equipment, even though
in the real world nobody has used a 2500 set in a business for
The source of echo problems on modern phone
systems is often the trunk, where a regular analog two wire
telephone line interfaces with the digital phone system - which is
"four wire," with separate transmit and receive paths
inside the system. You skip the four wire to two wire conversion
when you connect a T1 to the phone system, since both the T1 and the
phone system use separate transmit and receive (four wires).
There's a transformer inside every
analog telephone or CO trunk card who's purpose is to separate transmit and
receive. It's called a hybrid transformer. The hybrid has two
wires that connect to the trunk, and on the other side of the
four wires - a pair for receive (going to the receiver
in the handset in a standard phone), and a pair for transmit (going
to the microphone in the handset on a standard phone).
That hybrid is the source of
most echo problems that you can fix. The hybrid is basically two transformers
wound around a core, or an electronic version of that. Some hybrids are better than others. What makes
them better? They're more efficient, meaning that there is less
"leakage" between transmit and receive, or reflected audio back
out the two wire side.
On a regular 2500
set the hybrid in the network purposely sends some of the
transmitted signal from the mic to the receiver, which is called sidetone.
hybrid used on a trunk card is much more efficient than a 2500 set because the
sidetone isn't needed on a phone system (it's created at the station
so the conversation is more natural to the user).
Even though the hybrid on a trunk
card is efficient, it's not 100% efficient. Some of the transmitted
data "leaks" and gets mixed with the received data, or
reflected back to the caller on the 2 wire side. It's
at such low volume that the leakage just doesn't matter using an
analog or regular digital phone system.
The biggest problem with that
inefficiency on regular phone systems is that if you try to
amplify a two wire telephone
line at some point you'll get squealing due to the feedback from
transmit to receive, just like you'd get if a microphone is too
close to a speaker on a PA system. The feedback is why you don't see
amplifiers for two wire telephone lines, and why you don't see amplified
conferences on phone systems - at some point you'd get the
squealing, usually before you've amplified it loud enough to hear
So, you've got feedback on
an analog phone or phone system from the inefficient hybrid,
but it's never mattered until now because you can't hear it (it's
happening at almost the exact same time, so you don't hear it as
"echo"). That same hybrid is on the trunk card on digital phone systems
we've been using for years, and we've never had an echo problem (until
VoIP came along).
A digital phone system takes analog speech and runs it through a codec
(Coder-Decoder) that digitizes or "undigitizes" the audio. The codecs
on traditional digital TDM phone systems are so fast that there is very
little delay, and even though some of the transmitted data is mixed
with the received data in the hybrid, the delay is so small that we've
never noticed it (but it's there). You'd be hard pressed to hear any
difference between a call on an analog phone system and a digital
(TDM) phone system. VoIP is a different story...
When the Internet was created, it was
determined that packets of data would be much more efficient traveling
long distances than a single digitized stream of data (like TDM in a
digital phone system or on a T1). Packetization
means that the web page you're looking at could have gotten to your
computer through several paths across the country. Some
of the packets may have gone through Florida, and some may have gone
through California. The delays for each packet are normally much less
than 100ms (a tenth of a second) per segment between routers on the
Internet (it may take quite a few segments to get to you). Your
computer puts the packets together in the correct order so you see
everything in the right place on your screen. If it takes half a
second longer to get the top part of the web page than the bottom
part, nobody cares.
When people decided to put voice on
the Internet, they had to divide the conversation up into packets.
That's the only way the public Internet can get data from one
place to another. There have been many methods of digitizing voice
through the years. Until the Internet it really didn't matter
how it was done - the results were essentially the same. Unfortunately,
the only way to digitize voice and put it on the Internet is to
packetize it for the Internet using IP (Internet Protocol). While
these packets work great for data, which can arrive a little late or
in the wrong order, the whole concept works badly for voice over the
public Internet (but it works OK on a LAN or private WAN).
One of the troubles with creating
packets for IP is that it takes a while for the codec to do its work
- digitizing the voice and putting it in the packets. It's not a real long time, but it's long enough
for us to start hearing that inefficiency in the hybrid because
there's a delay. With analog or TDM, there was little or no delay
and we just didn't notice the inefficiency as echo, even though it
The better the hybrid matches the
phone line, the less
you'll notice echo on a VoIP phone system. If all phone lines were
exactly the same, the hybrid would be as efficient as possible.
Because all POTS lines are different in one way or another, the
hybrid is never as efficient as it could be.
Echo doesn't just effect voice. While
it's annoying to talk on a call with echo, there's a good
chance that an automated attendant or voice mail system won't work
correctly on calls with echo. The DTMF digits will look pretty
strange to the device hearing them. A 60ms DTMF digit followed by
50ms of an echo of that digit can cause recognition problems that
are tough to diagnose, especially if the echo is intermittent on
In most cases DTMF digits
aren't actually transmitted as audio from end to end over the Internet for VoIP,
they're sent as data. The line is split at the far end (the far end
doesn't hear that little piece of audio), and the digit is reproduced
at the other end of the call - throwing away the actual audio for that
digit. The person pushing the DTMF key hears the audio in their ear... but the
problem is that some of that tone is sent down the line to the
other end as an echo.
Echo problems can also occur
with analog station ports on a VoIP system since each port has a
hybrid transformer interfacing the analog phone or OPX, and there's a
delay involved in the codec that changes the four wire digital (IP)
side to the two wire analog side. Even with digital trunks, you may
still have echo on calls involving an analog station on a VoIP system.
Troubleshooting would be the same as on a POTS line with echo. In some
cases, you just won't be able to fix it, but if it's caused by an
impedance mismatch you can probably fix it with the
If it's Near End Echo, users
on the phone system won't hear the echo, but the caller on the other
end of the call will hear the echo. In that case, you need to call into
the system to hear the echo as you're trying to fix it (with echo
canceller settings in the system, attenuation, or the
Echo problems can also show
up only when the user is on a speakerphone when regular calls on the
handset aren't a problem. On VoIP calls if the transmitter in a phone
can hear the audio coming through the receiver, there's a good chance
you'll get echo - but if the user is on a handset the receiver is
usually sealed against their ear, and the transmitter never hears the
received audio (unless the user has a hole in his head?). On a
speakerphone it's very difficult to totally separate the transmit and
receive audio, so the design of the speakerphone is important as to
whether it will work OK on a VoIP system or VoIP phone line.
Fixing the echo problems...
These are the troubleshooting
steps to take:
Attenuate (reduce) the volume of the line
Loop current problems
Induced AC on the pair
Order a T1
If it's Near End Echo, users on the phone
system won't hear the echo, but the caller on the other end of the
call will hear the echo. In that case, you need to call into
the system to hear the echo as you're trying to fix it (with echo
canceller settings in the system, attenuation, or the Echo
1: If you can adjust the echo canceller
in the phone system from programming, see if that
helps. Setting the echo canceller too high will either do nothing,
cause clipping of one or the other party when both people try to
talk (kind of a ticking or buzzy sound), or create distortion so that the voices sound unnatural.
Even if a previous technician told
you that he adjusted the echo cancellers, check it anyway. You
can't skip any steps in the troubleshooting process. Assuming
anything can cause you to chase your tail for hours or even days.
2: If you can adjust the volume
of the phone line (the db level) in the phone system from
programming, see if that helps. Since the echo should be pretty far
in the background, reducing the volume of the phone line by a couple
of db might help. On the other hand, that 2db could be the
difference in being able to hear well on a phone call, and having
trouble hearing or being heard. Even if you don't leave it that way,
try reducing the db level to see if it gets you anywhere. The more
you know about your problem, the easier it will be to get it fixed
in the long run.
The only way to know whether
attenuating the line (lowering the level) will help the echo is to
put the attenuation in, and see if the echo problem improves
(without making it hard to hear).
Not all phone systems have
adjustments for the db level in programming, and sometimes reducing
the volume on the line before it gets to the hybrid
will help even when adjusting the gain in the phone system through
programming doesn't (because there's less audio to get reflected back from the inefficient hybrid).
Depending on the amount of
impedance mismatch, adding attenuation could correct echo in one
direction, but make it worse in the other (if the outside party
didn't hear echo before, you may have fixed the echo for the phone
system user but have made it worse for the outside party - or vice
You can reduce the gain using hardware.
Use regular 1/2 watt resistors, or
db attenuators made for reducing the level of a phone line
(which are basically resistors in box with a connector). The
pre-made attenuators come in 1, 2, 3, 4, 6 and 8db models. You
should never put more than 2db of attenuation in-front of a VoIP
system, since more attenuation than 2db could actually make the echo
worse in the other direction. Once you
determine how much attenuation you need with resistors, you can
order the attenuator you need, or just leave resistors on the line.
Note that a db attenuator is not
the same thing as a Loop Current Regulator or Attenuator,
which is specifically made to reduce the loop current (DC) without
lowering the db level of the line
(which is AC audio). Loop Current Regulators or Attenuators use
capacitors to block the DC while letting the AC audio through to the
phone system, so the user won't have problems hearing. For echo, you
want to attenuate the AC audio to see if you can get rid of the
Block db Attenuator
You can simply put resistors
in-series with both the tip and the ring to lower the db level. 100 ohm
resistors in-series with the tip and ring will lower the level of
the line by about 1db (be sure to use 1/2 watt or bigger resistors,
or they could burn up). Putting 220 ohm resistors in-series with the
tip and ring will bring the level down by about 2db, and it
also reduces the loop current by about 2ma.You can put resistors
in-series with each other, like two 100 ohm resistors in-series to
make 200 ohms of resistance.
You must always keep a telephone line
balanced, so whatever you put on the tip side has to be the same as
the ring side, or you'll create an imbalance (causing hum and
noise). Never put just one resistor on a phone line!
Don't let the loop current drop below
23ma. You must measure the loop current while you're
trying to attenuate the line to keep it from going below 23ma.
I suppose you can use resistors or attenuators
without knowing the actual circuit loss (db level) on the line, but
you shouldn't go below -7.5db of circuit loss from the CO or it will
be hard to hear.
If you want to test it and not guess
about how low the circuit loss is, you need the phone number for the
thousand cycle tone at the CO, and you need a circuit loss tester.
If you don't know the phone number of the 1KC tone at the CO, then
you need a 1KC tone generator so you can do a loop around test (send
from one line at the premise, and receive on another line). To test
circuit loss, you normally send from the CO at 0db, and measure what
you get at the premise. It has to be less than 0db
since there is always a loss as the audio travels over the copper
pair. Generally speaking, anything over -7.5 is OK.
Putting in 1 or 2db of attenuation
temporarily to see if it helps the echo is fine while testing. The
more you know about your problem, the better.
to 4db Loss Insertion Tester
You can use a Modular Loss
Insertion Tester if you don't want to mess with resistors during
lets you put 1, 2, 3 or 4db of loss in a line, depending on the
position of the two switches. Putting it in-series with a 4db
Modular Attenuator will let you insert 5, 6, 7 or 8db of loss in
a line (you'd only put that much loss on a very loud line, at a
premise that's very close to the CO),
but for VoIP you need to limit the attenuation to 1 or 2db.
3: If you can adjust the
impedance of the phone line in the phone system from programming,
see if that helps. The choices are usually 600 or 900 ohms.
Sometimes there's a jumper or switch on the trunk card to select
between 600 and 900 ohms.
Sometimes there will be a choice for
balanced or terminated. Terminated should be the first choice, but
if it doesn't work, try balanced. All you're trying to do with these
settings is match the impedance of the hybrid on the trunk card to
the impedance of the phone line.
IMPORTANT NOTE: A customer called and said that on the
Mitel systems he works on there's an Auto, 600 and 900 impedance
setting. None of those settings work for him on that system, but he
said he learned to set it at 600 ohms and let the Echo Stopper
match the impedance of the line to the trunk card in the system.
He said not to leave it on Auto!
He said it takes about 20 seconds after going off
hook for the echo to go away using the Echo Stopper if the system is
set to Auto.
If it doesn't tell you the
impedance is of the trunks in the manual, and tech
support doesn't know, there's no field test equipment that will give you
that information. Likewise, it's impossible to know whether the
phone line is actually 600 or 900 ohms.
Connecting a 600 ohm phone line
to a 900 ohm phone system (or vice versa) usually doesn't matter,
impedance mismatch can make the hybrid less efficient, causing
echo on a VoIP system. The echo effects the conversation, but
even more frustrating is the echo from an impedance mismatch that causes DTMF
recognition problems (that are difficult to diagnose/repair).
Since you can't measure the impedance
of the phone line or system, the only way to see if there's an
impedance mismatch is to change the impedance to see if the trouble
clears. The easiest way to do that is with the
Echo Stopper™ Adjustable Impedance
Matcher. This is as close to magic as it gets.
To use the
Echo Stopper™ you must
be able to reproduce the echo. Call in to the line you're working on
from another line, or call out on a system phone to another line. Put the Echo Stopper™ in-series with the phone line, and adjust the
black POT on top to see if the echo goes away. If not, push the Red
Range Switch, and again adjust the recessed pot. If the echo goes away, you need an Echo Stopper™ on
that line because you have an impedance mismatch. If it doesn't
help, you don't need it.
In the past, we've had a 600
to 900 Ohm Impedance Matcher to help get rid of echo problems, but it
wasn't all that effective. We finally figured out that phone lines are
seldom exactly 600 or 900 ohms. Likewise a phone system may not be
exactly 600 or 900 ohms. When we made the Impedance Matcher adjustable,
the echo just magically disappeared when we turned the POT. As you
adjust the POT, the impedance changes. On the lines we've tested it on,
the POT usually has to be turned quite a bit to get rid of the echo,
which demonstrates that we're not dealing with a clean 600 or 900 ohm
impedance, but somewhere above or below that. Don't forget to try the Red
Range Switch in both positions.
Unfortunately, there are more sources
of impedance problems that can make the hybrid less efficient,
causing echo. In some cases, the solution may be less expensive than an
Sometimes the line isn't terminated
at voice frequencies. A quick test is to bridge a 2500 set (on-hook)
butt-set in monitor position on the line to see if the echo
and/or DTMF problems go away. If they do, you can put a 600 ohm
resistor in-series with a .47mf capacitor, across the line. If you
don't want to mess with a resistor and capacitor, you can get a Half
REN Line Terminator.
Half REN Terminator
Half REN Terminator
Another cause of
impedance problems is misplaced load coils. Load
coils are inductors that are placed on a phone line at certain
intervals to boost the volume of the line. They are absolutely
necessary on long loops, or you wouldn't be able to hear. Unfortunately, the plant records
(which document where all of these
load coils are) are pretty
bad. Nobody knows where a lot of the load coils in this country are.
If they're on a line where they're not needed, they cause impedance
Load coils are normally added to
a whole binder in a cable
(25 pairs), so when an order for a new line goes to engineering they just pick
pairs that are already loaded, if load coils are needed. Sometimes
they run out of loaded pairs in a particular direction, and they add load coils
just to the pairs being installed. When that customer moves, those pairs
are reused - but often the records don't indicate that there are load
coils on the pairs, so the new customer gets a pair with load coils
that screw up the impedance of the line (because they're not
Load coils are normally placed on
long loops - about 18,000 feet or more. There are strict guidelines
for their placement. If they're put in the wrong place they change
the impedance of the line and cause volume, DTMF, or echo problems.
The first one has to be 3,000 feet from the CO, and the following
load coils are placed at 6,000 foot intervals. There must be 3 or
more load coils on the pair.
So what if there's a stray load
coil here or there? For speech on a 2500 set, it won't matter much. For a 56K modem, it will slow it down. It will probably
stop an ADSL modem from working. For calls answered by an automated
attendant, voice mail, fax server, or anything else that needs
hear DTMF digits - it may make digit recognition flaky. For speech
on a VoIP phone system, it causes echo because the impedance of the
line isn't correct, which makes the hybrid transformer less
Finding misplaced load coils is
very difficult. Trying the
Echo Stopper™ will let you adjust the impedance of the phone line to improve the impedance match. Seeing if the echo is fixed by the
Echo Stopper™ first makes sense.
If you have echo or DTMF problems, and
have checked all of the other fixes listed here, ask the phone
company for a meet to find out if there are mis-placed load coils.
You can ask them (in advance) to bring out a TDR (Time Domain
Reflectometer) because you suspect mis-placed load coils, but the
guy will likely show up without one. Maybe they'll bring it out on
the second meet? Maybe they have one?
The phone man will connect the TDR at
the premise, and look back towards the CO. The TDR will give him a
graphical representation of every splice, connection and half-tap on
the pair, as well as the first load coil. The TDR will show the feet
to each squiggle (connection), which the phone man can then walk or
drive to. The load coil has a specific squiggle (signature) on the
Some TDRs are able to see past the
first load coil to see other load coils on the line. The phone man
will be able to figure out the length of the pair from the CO, check
the records for that line to see if the pair should be loaded, and
see whether the load coils that are on the line are where they
should be - or if there's one or more that shouldn't be there at all.
Dealing with the phone company is probably as hard as it gets,
since you're depending on the phone company to confirm your
suspicion. The phone companies do OK at fixing a problem if you
diagnose it and tell them what's wrong, but they don't do well at
finding these kinds of problems themselves.
There are two types of TDRs. The most
common type can only see to the first load coil from the premise.
Riser Bond sells one that will see all of the load coils on the
line. Neither are cheap. Riser Bond is at 800-688-8377. I don't
think you can rent TDRs that will see past the first
load coil, but if you just need to see to the first load coil, I
think those are available at communications equipment rental
4: Check the loop current on the line. High
loop current will cause the components on the trunk card to run hot.
Sometimes hot enough to damage them, and sometimes hot enough to
make the components so warm that their value is outside the range
that the engineer was expecting when he designed the circuit. Maybe
they'll be louder than normal, which would increase the echo?
Running the system with high loop
current means you're now the first one to test the phone system to
see if it works at that high of a loop current. The normal range for
loop current is 23 to 35 ma
(some telephone equipment doesn't like
anything higher than 27ma).
Low loop current also
causes strange things to happen when the components run out of spec,
so it's definitely worth checking the loop current. You can check it
with any digital meter that has a range for DC ma. Be sure you set
your meter to DC current, be sure you use the correct banana jack
(the + lead often goes in a different jack for current
than for voltage), and you are connecting the meter in-series
with the tip or ring so the electricity is flowing through
You'll read about 0ma (or maybe .01ma) when you're on-hook
and the actual loop current when off-hook. You can
only go off-hook with one telephone for testing, don't go off-hook
with the phone system and your butt-set at the same time or
your reading will be meaningless!
The Loop Current Tester is
easier to use than a meter. Just put it on a line, press the button,
and an LED will light indicating a range for the loop current (from LOW
5: Check for Induced AC on the pair (see
DTMF Problems a couple of paragraphs down). Generally
speaking anything over .5VAC from tip to ground, ring to ground or tip to ring
can cause problems with any kinds of tones on the line (modem, fax, DTMF). It
generally won't effect a simple 2500 set or Butt-Set, but anything that's
plugged into an AC outlet like a modem, fax or phone system can get really
screwed up by induced AC on the line.
That's all you can do. It's time to get a shiny new T1 and hope
that fixes it!
While an impedance mismatch, loud audio or high loop current can
screw up DTMF and modem / fax tones (see the fixes for echo, above), an easy thing to check for and the most common
cause of DTMF problems is induced AC on a
With induced AC, the 60 cycle waveform with the two other DTMF tone
waveforms looks like a third tone, making DTMF recognition
unreliable (it also screws up Caller ID). Our
Longitudinal Imbalance Tech
Bulletin tells you how to test to see if there is more than
.5VAC from tip to ground or ring to ground (which is usually the
threshold where problems start), and how to fix it if there is
(there should never be any AC voltage on a phone line except during
ringing, but 1/2 a volt is usually OK).
If the problem is intermittent the LD carrier (long distance carrier) for the
phone line calling a Voice Mail or IVR can be killing the DTMF digits,
after the call is connected. The carrier can be compressing the voice/DTMF,
or the DTMF digits can be too loud or otherwise distorted.
The easiest way to see if the problem is with a particular carrier is to use
a 1010 Dial-Around code, which lets you choose a deferent LD carrier than the
line is PIC's to (won't work with VoIP, cable, etc.). I would start with the
1010288 AT&T code to see if the DTMF digits are recognized by the machine.
If the IVR or Voice Mail has 800 numbers pointing to POTS lines, try dialing
the POTS number instead of the 800 number, to see if the 800 number provider is
causing the DTMF problems. If that works, try dialing hte POTS number using
For more info on 1010 Dial Around codes see our
October 2014 Tech Blog.
Look at the Longitudinal
Imbalance Tech Bulletin for induced AC causing DTMF problems.
to Think About Before Ordering VoIP for a detailed look at VoIP quirks.
Look at the
Checklist for a quick look at things to check with VoIP in a
home or business.
& Fixing Telephone Lines to fix other phone line problems.