Fax over IP not working? A T.38 troubleshooting checklist

The office moved to fibre, the phone company swapped the old line for an internet box, and now the fax machine that worked for fifteen years fails every second page. Or the recipient of your fax keeps telling you "nothing arrived". Faxing over an IP network is possible, but it is more delicate than voice. This article explains why and gives you a checklist, in the order that usually saves time.

Why VoIP and fax clash out of the box

A fax machine speaks by whistling. It sends audio tones that carry the page as a stream of tiny changes in pitch and phase. On a copper line that sound travels untouched. Over an IP network, the call is cut into small packets, compressed, sent separately and reassembled at the other end. Voice survives this very well, because human ears forgive a lost millisecond. A fax modem does not forgive it. Three kinds of damage account for most failures:

• Compression. Codecs such as G.729 or Opus throw away detail that matters for speech and that a fax needs. Even the standard G.711 is only safe if nothing else tampers with it.
• Jitter and packet loss. Packets arriving late or out of order produce gaps in the tones. A few are tolerable, a burst is not.
• Clock drift and echo handling. Voice equipment applies echo cancellation and silence suppression that can mistake fax tones for noise and cut them.

Carrying a fax over IP: two methods

Pass-through. The fax tones travel inside an ordinary voice channel, usually in G.711. It works on a clean, low-latency link and fails quickly when the network gets busy.

T.38. A standard that turns the fax into data messages before they cross the network and rebuilds the tones at the other end. Because the tones never travel as audio, compression and moderate packet loss do not wreck them. T.38 is the right answer when both ends and every device between them support it. A longer explanation is on our T.38 page, and the under the hood page describes how CaptainFax approaches the call on its side.

The catch: T.38 must be supported and enabled by the line provider and by your adapter, and the two must negotiate it successfully. When they do not, the call quietly falls back to pass-through and the trouble starts.

The checklist

Work through these in order. After each change, try one single-page test before moving on.

1. Confirm it really is a fax problem. Ring the number with a normal phone. A fax machine answers with a short, high-pitched beep after a few seconds. A person answering means you have the wrong number, which is the commonest cause of all.
2. Check the line type. Some internet boxes and business VoIP plans do not provide a fax-capable line at all, or provide it only as an option. Ask your provider whether T.38 or fax pass-through is supported on your line.
3. Use G.711 only. On your adapter or phone system, force G.711 (A-law in Europe, mu-law in North America) for the fax line and remove G.729, Opus and other compressed codecs from the list. This single change fixes a large share of problems.
4. Turn T.38 on, then test both ways. If your adapter has a T.38 option, enable it. If calls get worse, switch to pass-through and try again; some provider combinations negotiate T.38 badly.
5. Disable echo cancellation, silence suppression and comfort noise on the fax port. These features are helpful for voice and harmful for fax.
6. Lower the speed. Set the maximum modem rate to 9,600 bit/s, or 14,400 at most, instead of 33,600. A slower transmission has far more tolerance for jitter. It adds a few seconds per page, which is rarely important.
7. Look at ECM. Error Correction Mode lets two machines resend damaged blocks. Leave it on if the line is stable. If it causes endless retries on a poor link, test with it off, and note that without it a bad block shows up as a stripe or a distorted line on the page.
8. Remove the jitter buffer surprises. Set a fixed jitter buffer rather than an adaptive one if the option exists, around 60 to 100 ms.
9. Plug the adapter straight into the router. A switch with a busy download behind it is enough to disturb a call. If the router supports it, give the adapter's traffic priority (QoS).
10. Avoid VPNs and double NAT between the adapter and the provider. Both add delay and can break the signalling.
11. Update the firmware of the adapter and the router. Fax behaviour is a frequent item in release notes.
12. Test at another time. If it works at 7 a.m. and fails at 3 p.m., you have a congestion problem, not a settings problem.

When the problem is at the recipient's end

If you send with CaptainFax and the tracking page shows failure on a particular destination, the cause may be in the recipient's line rather than yours. Many offices have moved their old fax number to an internet line without checking that it still works for fax. You cannot change their configuration, but you can give them a precise request: "please check that your fax line supports T.38 or G.711 pass-through, and that echo cancellation is off". Print the checklist above for them if it helps. Try again at a different time of day and, where the destination allows, with a short test page first. Our answers page lists other causes of failed sends.

Remember too that landlines only are supported, and that a number which now rings an internet box can behave differently from the copper line it replaced.

Quick reference

FAQ

Is T.38 always better than pass-through?

Not always. T.38 is more robust when both ends support it properly, but a poor implementation can do worse than clean G.711. Test both.

Should I keep ECM switched on?

Usually yes. It resends damaged blocks instead of printing them. Turn it off only as a diagnostic step if the link keeps stalling.

Can I avoid all this by not owning a fax line?

If you only need to send, yes: you upload the document and no machine of yours is involved. The rates page explains the prices.