Dialling in through a telephone adapter
If your telephone line is really an adapter plugged into your internet, a modem will often ring, answer, and then fail with no useful error. Here is why, and the five settings that usually fix it.
Why it happens
A modem does not send data. It sends sound, and it needs that sound to arrive with its shape intact.
Every codec a VoIP provider prefers is a speech codec. It models a human voice and throws away everything that does not fit that model. Modem tones do not fit that model. The result is not a slow connection: it is a handshake that never completes, over and over, with nothing in the error message pointing anywhere useful.
G.711 is the only codec that works. It is uncompressed, and it is what the old telephone network itself used. Anything else and the call will ring, answer, and fail.
The five settings
The names differ between adapters. These five do not. All five, or it will not train.
1. Codec: G.711
Pinned, not merely preferred, so nothing can renegotiate to something else part way through the call. In North America that is G.711 u-law.
2. Echo cancellation: off
It is built to remove exactly the kind of steady, correlated signal a modem handshake is made of.
3. Voice enhancement: off
Noise suppression, comfort noise, silence suppression, VAD, AGC, anything called "clarity". Every one is a filter that assumes speech.
4. Passthrough on, T.38 off
T.38 is a fax protocol. It takes the carrier apart and rebuilds it at the far end, which is fine for fax and fatal for a data call. You want plain G.711 passthrough.
5. Jitter buffer: fixed and small
An adaptive buffer changes depth during the call, and a modem reads that as the line jumping. Fixed.
Still nothing?
Some adapters simply do not expose these, and some providers override them. That is a real dead end and not your fault.
Tell us what you have and we will say whether we have seen it work.
Exact settings, measured on our own bench
The five principles above are the general case. Below is the literal, to-the-letter configuration we run on the same hardware here, with every value checked against a real modem rather than a manual.
Grandstream HT801 / HT802 / HT812 / HT814
All of these live under Profile 1 except the SLIC setting. Apply them, click Update, then Apply, and confirm the port still shows Registered.
| Setting | Value | Why |
|---|---|---|
| Preferred Vocoder | PCMU (then PCMA) | G.711 only. Remove G.722, G.729, iLBC, OPUS, GSM and G.726 from the list entirely so none can be negotiated mid-call. |
| VAD | No | Silence suppression cuts the carrier the moment it looks like a pause. |
| Fax Mode | Pass-Through | Never T.38. |
| Re-INVITE After Fax Tone Detected | Disabled | A mid-call re-INVITE renegotiates media and drops the carrier. |
| Disable Line Echo Canceller (LEC) | Yes | The wording is a double negative: Yes means the canceller is OFF, which is what you want. |
| Disable Network Echo Suppressor | Yes | Same double negative, same reason. |
| Jitter Buffer Type | Fixed | Adaptive resizes during the call and the modem reads that as the line moving. |
| Jitter Buffer Length | Low | Low, not Medium. See the warning below: this one is counter-intuitive and we measured it. |
| Voice Frames per TX | 1 (10 ms) | Halves packetisation delay, and halves the size of any glitch if the buffer does slip. |
| Enable Session Timer | No | Otherwise the adapter re-INVITEs every few minutes, which can kill a long session. |
| Register Expiration | 3600 | Re-registering every minute is needless churn. |
| SRTP Mode | Disabled | |
| SLIC Setting (FXS Ports) | USA 1 (Bellcore 600 Ω) | North America. |
Do not raise the jitter buffer to help a marginal line. It is the obvious move and it backfires. We tested Low against Medium with the arms interleaved: one notch up added roughly half a millisecond of path delay, which was enough to push the negotiation onto a lower symbol rate and cost 2,400 bps on every single connection (13 of 13 calls at the top rate on Low, 2 of 12 on Medium). Putting it back restored the rate immediately. Minimise path delay; never add buffer for safety.
Your modem
Start from factory defaults and add hardware flow control. Most trouble on an adapter is the adapter, not the modem.
AT&F restore factory defaults
AT&K3 hardware (RTS/CTS) flow control
ATX4 full result codes, so BUSY and NO DIALTONE are distinguishable
ATM2 speaker on for the whole call, while you are diagnosing
AT&W save itDo not cap the speed low as a permanent setting. On a line carrying G.711 cleanly, a low symbol rate is not the safe choice. It is the failure mode. In our own measurements the lowest V.34 symbol rate almost never completed a connection while the highest one almost always did. Use a cap to diagnose, as described below, then take it off.
External US Robotics modems
The DIP switches are read only at power-on, so change them with the modem off and then switch it on. The legend is printed on the underside of the case, and the up/down convention differs between models, so go by the label rather than by a number you read somewhere.
- Switch 1, DTR: normal for ordinary use. Set it to override / always on only if the modem must work without a computer asserting DTR.
- Switch 5, auto answer: off for dialling out.
- Switch 8, command recognition: must be in smart mode, or the modem silently ignores every AT command.
If you connect one by serial and it never answers an AT command, suspect the cable before the modem. A null-modem cable will not work: it crosses transmit and receive, and the modem never hears you. You need a straight-through modem cable, and the giveaway is that the ring-indicator line still works while nothing else does.
Working out whose end it is
Listen to the call
Turn the modem speaker on and leave it on for the whole call, then dial as usual:
ATM2
ATDT19405483638- Answer tone, then a handshake, then silence. That is a good call. The speaker cuts once the two modems agree.
- Answer tone, then noise that never settles. That is the codec mangling the sound. Work through the five settings above.
- Nothing at all after it answers. The call is probably not reaching us. Check the number, and any digit you need for an outside line.
- Compressed, warbly or gated speech when you ring any recorded announcement. No modem will train over that path, whatever else you change.
Try a slower connection first
A 2,400 bps call survives a marginal line that a 14,400 bps call will not. Capping the modem is often the difference between a connection and none:
AT+MS=2,0,1200,2400If that connects and full speed does not, the line is the problem rather than the modem, and the five settings are where to look.
The trade-off is plain: this caps you well below what a V.32bis modem can do on our faster numbers. Use it to find out whether the path works, then raise it again.
What we do at our end
Worth knowing, because it rules out half the possible causes. Our own trunk is locked to G.711, we do not run echo cancellation or any speech processing on the answering path, and we do not use T.38 anywhere. If the sound is being altered, it is being altered between your modem and us.
A telephone adapter that cannot be made to pass a modem is a real limitation of this service and we would rather say so than have you keep trying. Ask us before you spend an evening on it.