Can my fax machine work on a fibre connection?

Yes: fibre itself is not the issue, the modem or ATA is. With clean G.711 passthrough or T.38, and the machine capped at 9,600 bps, most transmissions go through.

Should ECM be off when the line uses T.38?

Not necessarily. ECM mainly causes trouble on G.711 passthrough. If your line genuinely negotiates T.38, leave it on, run a series of tests, then compare with a series with ECM off.

How do I know whether my modem supports T.38?

Look for a fax or T.38 option in the phone settings of the modem's interface. If there is none, ask your provider's support: the answer often depends on the model and firmware.

Fax over IP: connecting a fax machine to a modem or VoIP line

A configuration guide for keeping a fax machine working behind fibre, cable or an IP phone system.

A fax machine works behind a modem or VoIP line as long as its call is neither compressed nor delayed along the way. Two techniques make that possible: G.711 passthrough, which carries the signal like raw voice, and T.38, which turns it into data. Cap the machine at 9,600 bps, turn off ECM on G.711, then test with several pages.

Key points

Two modems talk to each other for the whole transmission: anything that distorts the audio (compression, packet loss, jitter) breaks the handshake or the page in progress.
T.38 is the method designed for IP networks; G.711 passthrough also works, but only on a clean connection with no transcoding.
Starting point on the machine: V.29 at 9,600 bps, V.34 (Super G3) off, ECM off unless the line really uses T.38.
A single-page test proves nothing: validate with a multi-page document at different times of day.

Why faxes fail on an IP line

On copper, a fax machine had a continuous circuit with a constant delay. Behind a modem, the signal is digitized, cut into packets of 10 to 30 milliseconds and sent across a shared data network. The T.30 fax protocol was never built for that: it expects specific tones at specific moments.

Three things cause most failures. Compressed codecs (G.729, Opus, AMR) are tuned for speech and discard exactly the detail the modem uses to encode its data. Jitter, meaning packets arriving at uneven intervals, forces the device to stretch or clip the stream. And a single lost 20 ms packet at the wrong moment can wreck a scan line.

On G.711, ECM (Error Correction Mode) often makes things worse: it asks for every damaged block again, retransmissions pile up, the session runs out of time and the machine hangs up. Without ECM, a lost packet at worst leaves a white streak, and the page stays readable.

T.38 or G.711 passthrough: the two methods side by side

The modem, adapter or gateway picks the method when the call is set up.

T.38 or G.711 passthrough: the two methods side by side
Criterion	T.38 (fax relay)	G.711 passthrough
How it works	The fax signal is demodulated where it enters the IP network, sent as data (UDPTL), then remodulated at the far end.	The modem audio travels as is, like an uncompressed phone conversation.
Packet loss	Handled well: each packet repeats the previous ones (redundancy).	Poorly tolerated: one loss can damage a line or stop the page.
Jitter	Little effect, since the modem timing is rebuilt locally.	Strong effect; a fixed jitter buffer helps a lot.
Bandwidth	Modest, rising with the redundancy level chosen.	About 64 kbps of audio each way plus IP headers, so 80 to 90 kbps.
Requirements	Modem or ATA, provider and receiving gateway must all accept the switch to T.38 (SIP re-INVITE).	G.711 end to end (μ-law in North America), with no transcoding.
Recommended speed	Up to 14,400 bps (V.17) often works; avoid V.34.	9,600 bps (V.29), ECM off.

Settings on the machine and on the modem

Menu names vary by model; treat these values as a starting point.

Modem speed: Limit transmission to V.29, which means 9,600 bps. Starting at 14,400 bps leads to more failures over IP.
Super G3 / V.34: Turn it off: at 33,600 bps it needs an almost perfect line.
ECM: Off on G.711. If T.38 is working properly, turn it back on and compare.
Modem phone port: Use the analog port on the modem (often labelled Phone or Tel), directly, with no splitter or old DSL filter. Mark it as a fax port if the interface offers that.
Codec and voice features: If your modem or ATA allows it: G.711 μ-law first, with silence suppression, comfort noise and echo cancellation off.
Jitter buffer: A fixed buffer of 60 to 100 ms rather than an adaptive one, which resizes itself mid-call.

ATA, gateway or fax server: which equipment

In Canada, a home or small-office landline now usually reaches the premises through a provider's modem or hub: Bell or Telus over fibre, Rogers or Videotron over cable. Most have an analog phone port, but how they handle a fax call (passthrough, T.38, adjustable settings or not) depends on the model and firmware. Check your modem's interface, then ask your provider, before buying any hardware.

An ATA (analog telephone adapter) turns an analog port into a SIP account, which is useful when a business uses a VoIP provider or an IP PBX rather than the provider's modem. Pick a T.38-capable model and confirm the SIP trunk accepts T.38 as well; otherwise the call falls back to G.711.

A multi-port FXS gateway keeps several fax machines running at one site. A fax server replaces the machines: it delivers incoming faxes as PDFs to mailboxes and sends from desktops, but it needs a T.38 trunk and someone to run it.

Validate the setup before calling it reliable

First check that a phone plugged into the same port gets a dial tone and can reach a landline. If not, the problem is the line, not the fax.

Then send a five-to-ten-page document mixing fine print and dark areas to a contact who will tell you what came out. Repeat at three different times, including one in the evening when your connection is busiest, and ask for a fax back to test receiving.

Keep the transmission reports: a machine that keeps dropping to 4,800 bps points to a fragile line, even if the pages get through. If failures continue, open a ticket with your provider and ask whether your line supports T.38.

When to switch to an online fax service

Keeping a fax machine behind a modem makes sense if you regularly receive faxes on a number people already know. If you only send, a few times a month, the setup and upkeep soon outweigh the machine itself.

An online service then removes the IP chain on your side: you upload the PDF in your browser and the service calls the recipient's fax machine. FaxTelecom works that way, with no account, to landlines in seven countries including Canada. It provides no number and receives nothing, so it complements a receiving line rather than replacing it.

Fax a PDF from your browser

Fax over IP: frequently asked questions

Can my fax machine work on a fibre connection?: Yes: fibre itself is not the issue, the modem or ATA is. With clean G.711 passthrough or T.38, and the machine capped at 9,600 bps, most transmissions go through.
Should ECM be off when the line uses T.38?: Not necessarily. ECM mainly causes trouble on G.711 passthrough. If your line genuinely negotiates T.38, leave it on, run a series of tests, then compare with a series with ECM off.
How do I know whether my modem supports T.38?: Look for a fax or T.38 option in the phone settings of the modem's interface. If there is none, ask your provider's support: the answer often depends on the model and firmware.

Telephone network

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Hand-picked pages to help you prepare, send or prove your fax.

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