Your office switched to fiber, the cable company swapped the old phone line for a VoIP service, and now the fax machine that ran for fifteen years fails every other page. Or the person you are faxing keeps saying "nothing came through." Faxing over an IP network works, but it is touchier than voice. This article explains why and gives you a checklist in the order that usually saves the most time.
Why fax and VoIP don't get along by default
A fax machine communicates by whistling. It sends audio tones whose pitch and phase carry the page. On copper, that sound arrives untouched. Over an IP network, the call is chopped into packets, compressed, sent separately and reassembled. Voice survives this because human ears forgive a lost millisecond. A fax modem does not. Three kinds of damage cause most failures:
- Compression. Codecs such as G.729 or Opus discard detail that matters for fax. Even G.711, the standard uncompressed choice, is only safe when nothing else alters it.
- Jitter and packet loss. Late or out-of-order packets leave gaps in the tones. A few are tolerable. A burst is not.
- Echo handling and silence suppression. Voice gear can mistake fax tones for noise and cut them.
Two ways a fax can travel over IP
Pass-through. The tones travel inside an ordinary voice channel, usually G.711. Fine on a clean, low-latency link; it fails fast when the network gets busy.
T.38. A standard that converts the fax into data messages before they cross the network and rebuilds the tones at the far end. Since tones never travel as audio, compression and moderate packet loss do less harm. It's the right answer when both ends and everything between them support it. For more detail see our T.38 explainer; the under the hood page describes how CaptainFax handles the call on its side.
The catch: your provider and your adapter must both support T.38, and they must negotiate it successfully. When they don't, the call silently falls back to pass-through, and that is where trouble starts.
The checklist
Go in order. After each change, send a one-page test before moving on.
- Make sure it's a fax problem. Dial the number from an ordinary phone. A fax machine picks up with a short, high-pitched tone. If a person says hello, the number is wrong, which is by far the most frequent cause.
- Check the line type. Certain residential VoIP plans lack a fax-capable line, or offer one only as an extra. Ask your provider if T.38 or fax pass-through is supported.
- Stick to G.711. On your analog telephone adapter (ATA) or phone system, force G.711 on the fax port (mu-law in the US and Canada, A-law in Europe) and strip out G.729, Opus and other compressed codecs. This single change resolves a lot of cases.
- Turn on T.38, then test. When your ATA has a T.38 setting, enable it. If calls get worse, revert to pass-through, since some provider and adapter combinations negotiate it poorly.
- Switch off echo cancellation, silence suppression and comfort noise on the fax port. They help voice but hurt fax.
- Slow it down. Limit the modem to 9,600 bit/s, or 14,400 at the most, rather than 33,600. A slower transfer copes with jitter much better and adds only seconds per page.
- Review ECM. Error Correction Mode allows the machines to resend damaged blocks. Leave it enabled on a stable line. If it triggers endless retries on a weak link, try it disabled, remembering that a bad block then shows up as a streak on the page.
- Set the jitter buffer. Where you have the choice, take a fixed buffer of around 60 to 100 ms instead of an adaptive one.
- Plug the ATA directly into the router, rather than into a switch shared with a bandwidth-hungry device. Turn on QoS for the adapter if your router has it.
- Keep VPNs and double NAT out of the path between adapter and provider. Each adds delay and can break signaling.
- Install firmware updates for the ATA and router. Release notes often mention fax behavior.
- Test at another hour. If it works at 7 a.m. and fails at 3 p.m., the culprit is congestion, not configuration.
When the problem is at the recipient's end
If you send with CaptainFax and the tracking page shows a failure to one particular destination, the cause may be the recipient's line. Many offices abroad moved their old fax number to an internet line without checking that it still handles fax. You can't change their setup, but you can ask precisely: "Please check that your fax line supports T.38 or G.711 pass-through and that echo cancellation is off." Feel free to share the checklist above. Try again at another hour, with a short test page first. Our answers page lists more causes of failed sends.
Landlines are the only supported destination type, and a number that now rings an internet box may behave differently from the copper line it replaced.
Quick reference
| Symptom | Likely cause | First thing to try |
|---|---|---|
| Call connects, then drops | Codec or T.38 negotiation | Force G.711, retest |
| Streaks or garbled lines | Packet loss, no ECM | Enable ECM, lower speed |
| Works at night, fails by day | Network congestion | QoS, wired connection |
| Never connects | Wrong number or no fax support | Call by voice, ask provider |
| Pages cut off midway | Timeouts at high speed | Cap at 9,600 bit/s |
FAQ
Is T.38 always better than pass-through?
No. T.38 is more robust when both ends implement it well, but a poor implementation can do worse than clean G.711. Test both.
Should I leave ECM on?
Usually. It resends damaged blocks instead of printing them. Turn it off only as a diagnostic step if the link keeps stalling.
Can I skip all this if I only send faxes?
Yes. Upload the document and no machine of yours is involved. See the rates page for pricing.
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