My fax won't go through: diagnosis and fixes in 2026

Short answer: nine fax failures out of ten come down to just four causes — a badly formatted number (country code, leading zero, stray spaces), a document the protocol can't handle (file too large, colour image, password-protected PDF), a degraded line (misconfigured VoIP, no T.38, an end-of-life PSTN line) or a recipient who simply no longer has a working fax machine. The right reflex isn't to resend ten times: it's to isolate the cause with three tests, then act.

About MondialFax. This guide is a general troubleshooting manual, valid whatever your sending method (physical fax machine, all-in-one printer, online service). MondialFax sends faxes free of charge and without registration to seven destinations (France, Belgium, Germany, Spain, Italy, Portugal, Canada). It does not assign a number and does not handle reception: the reception scenarios mentioned below assume a third-party provider. See receiving a fax and our fax-to-email guide.

There's a particular kind of exasperation, unique to fax, that email has never managed to produce. An email that won't send tells you plainly: "address not found". A fax, on the other hand, just keeps going. It dials, it whistles, it waits, it redials, and after twelve minutes it spits out a terse report — "transmission failed" — without explaining anything at all. You've lost a quarter of an hour and you still don't know whether the problem is you, the network, or the office across town that threw its fax machine in the skip back in February.

This article proposes the opposite approach: instead of retrying, diagnose. With three tests and a table mapping out error messages, you can identify the real cause in almost every case.

The three tests that pinpoint the fault in ten minutes

Before any technical fiddling, run these three checks in order. They separate the three broad families of causes: you, the line, the recipient.

Test 1 — Send to a number you're sure about

Send a single page to a fax number you know is active: a colleague, a government office, your own receiving number if you have one. If that fax goes through, your equipment and your line are working: the problem lies with the recipient or with the number dialled. If that fax fails too, the problem is at your end.

Test 2 — Call the fax number from a phone

This is the most underrated and most informative test. Dial the recipient's number from an ordinary phone and listen:

• A shrill, continuous tone → there is indeed a fax machine at the other end, ready to receive. The problem lies elsewhere.
• A ringtone that leads to a human voice → the number is no longer a fax number. This happens on a massive scale since the gradual PSTN shutdown: the line has been renumbered or reassigned.
• "The number you have dialled is not in service" → the line has been cancelled. No setting will save this transmission.
• Ringing into the void, with no answer → fax machine switched off, out of paper, or automatic answering disabled.

Test 3 — Change the document

Try the same number again with a minimal document: a single page, black text on a white background, no logo or photo. If that page goes through when your original document failed, the cause is the file, not the line. This is extremely common and we'll come back to it below.

Cause no. 1: the number is badly formatted

This is the leading cause of failure by far on international transmissions. Fax has zero tolerance: unlike a smartphone that "guesses" formats, a fax machine or gateway dials exactly what you give it.

The rules to remember:

The universal rule: for international calls, drop the leading zero of the national number and replace it with the country code. This one detail causes tens of thousands of failed transmissions every year. Our guide on sending a fax abroad breaks down the covered destinations country code by country code.

Another trap: stray characters. A number copied from a website or a PDF often drags along non-breaking spaces, dots, em dashes or a "Fax:" prefix that the gateway can't clean up. Retype it by hand, digit by digit. If you handle a steady stream of correspondents, a simple up-to-date professional address book — or a paper telephone directory kept next to the machine — avoids the sloppy transcription that causes tomorrow's failure.

Cause no. 2: the document doesn't clear the protocol

Fax is a 1980s standard (Group 3, ITU-T recommendation T.30). It transmits a black-and-white image, line by line, at speeds that in practice top out around 14.4 kbit/s. Anything that departs from that model causes trouble.

The most frequent document-related causes:

• The PDF is protected. A file with a password or printing restrictions cannot be converted into an image. Remove the protection before sending.
• The document is too heavy or too long. One fax page represents roughly 30 to 60 seconds of transmission. A 40-page file means a 25-minute call — the slightest micro-dropout makes the whole thing fail. Split it into batches of 5 to 10 pages.
• The scan is in colour or high-resolution greyscale. Fax converts them to pure black and white; grey fills turn into illegible black blocks or inflate the transmission time. Scan in black and white, 200 dpi.
• The scan quality is poor. Photographing a document with a phone, at an angle, on a dark desk produces a file that fax renders unusable. A portable document scanner or, failing that, a scanning app with automatic deskewing radically changes the result.
• The document uses very fine fonts or coloured stamps. Below 8 points, fax output becomes a mush of pixels.

A word about paper, for those still using a physical fax machine: repeated jams almost always come from paper that's too thin, damp, or from a clogged automatic feeder. A pack of 80 gsm A4 paper stored flat, away from humidity, plus a clean of the rollers with roller cleaning wipes settles most mechanical incidents.

Cause no. 3: the line is degraded (the big 2026 topic)

This is where the most baffling failures concentrate — the ones affecting transmissions that "worked perfectly well last year".

Orange stopped selling new PSTN lines in November 2018, then began the technical shutdown region by region from 2023, under Arcep supervision. As a result, your fax machine is increasingly plugged in behind a router, on an emulated analogue port. And fax and VoIP get along badly.

Why VoIP breaks fax

The T.30 protocol assumes a continuous channel, with no compression and no loss. IP telephony does exactly the opposite: it compresses the voice (G.729, G.722 codecs), strips out silences, and tolerates a few lost packets — imperceptible to a human ear, fatal to modem modulation.

The typical symptoms:

• Transmission starts then cuts out at page 2 or 3.
• The report says "communication error" or shows a code such as #0018 / #0022.
• Pages arrive streaked with horizontal black bands.
• The fax goes through one time in five, with no apparent logic.

The fixes, in order:

1. Force the G.711 codec (a-law in Europe) on the line: this is uncompressed voice, the only codec genuinely compatible with fax.
2. Enable T.38 on the operator's side if available — a protocol designed to relay fax over IP. Our article fax over VoIP: why T.38 fails covers the cases where it isn't enough.
3. Reduce the fax machine's transmission speed to 9,600 bit/s in the advanced settings. It's counter-intuitive, but a slower rate is markedly more robust.
4. Disable ECM (Error Correction Mode) if failures persist: ECM demands retransmission of corrupted blocks and eventually gives up when loss is chronic.
5. Plug the fax machine directly into the router's dedicated port, never behind an extension lead, a splitter or a leftover ADSL filter.

On that last point: a good-quality, short shielded RJ11 telephone cable is a worthwhile replacement for the original cord that's been kinked behind a cabinet for fifteen years. This isn't a trivial detail — a damaged cord introduces enough line noise to bring down a modem negotiation.

The case of lines still running on copper

If you're still on pure PSTN, failures may stem from attenuation on the copper pair, which is barely maintained these days. An entry-level telephone line tester lets you check for voltage and signal quality in thirty seconds before calling support. If the line is the culprit, the real answer in 2026 is no longer repair: it's migration. See keeping your fax number after the PSTN shutdown.

Decoding error messages

The wording varies from one manufacturer to another, but the families are stable.

Always keep the transmission log: it's your only proof of sending in the event of a dispute, and the starting point for any serious diagnosis. On the value of that evidence, read the legal value of fax in 2026 and archiving your faxes.

When the problem is the recipient

This is the conclusion nobody likes to reach, and yet it's becoming the most common one. Since 2023, many practices, court registries, insurers and suppliers have unplugged their fax machines without updating their legal notices, their letterheads or their websites. The number is still displayed; the machine no longer exists.

A few unmistakable signs:

• The number rings into the void consistently, at any hour, several days running.
• It leads to a voice menu or a reception switchboard.
• The organisation now points to an online document upload portal, a secure healthcare messaging system or an encrypted email address.

In that case, the only useful move is to phone up and ask which channel is now in use. For healthcare exchanges, secure messaging has largely replaced fax — we covered this in fax and healthcare in 2026.

Switching to online sending: what it fixes, what it doesn't

Sending from an online service instantly removes an entire category of faults: no more paper jams, no more empty cartridges, no more full memory, no more noisy analogue line, no more unstable modem negotiation — the operator's gateway handles all of that on infrastructure built for the job. It's the quickest shortcut when you have a single urgent transmission to make and a temperamental fax machine.

What it doesn't fix, on the other hand: a wrong number stays wrong, a recipient with no fax machine stays unreachable, and a badly scanned document will arrive badly scanned. The quality of the source file remains your responsibility — hence the value of a clean scan upstream.

For a one-off transmission, you can send a fax online to one of the seven covered destinations, with no registration. From a phone, the procedure is described in sending a fax from your smartphone.

Frequently asked questions

Why does my fax always fail on the same page?

Almost always a line problem, not a document problem. A drop-out that consistently occurs around the same call duration points to an operator-side timeout or regular packet loss on a VoIP link. Split the transmission into shorter batches and reduce the sending speed to 9,600 bit/s.

How many times should I retry before giving up?

Three attempts spaced at least twenty minutes apart are enough to rule out a simple busy line. Beyond that, retrying brings no new information: move on to the phone test of the number.

My fax goes through but arrives black or illegible at the recipient's end. Why?

In order of frequency: document scanned in colour or greyscale, resolution too high, contrast badly set, or a dirty scanner glass. Rescan in black and white at 200 dpi and compare.

Does free fax work less well than paid sending?

No: the transmission uses the same gateways and the same T.30 protocol. What changes is the destinations covered, the permitted volume and the extra features (detailed confirmation, reception, dedicated number). Transmission quality itself depends on the recipient's line, not on the price paid to send.

Can I know whether the recipient actually read my fax?

No. The confirmation confirms that the pages were received by the machine, not that a human read them. That's a crucial distinction in a dispute: the log proves delivery, never that the content was read.

In summary

• Three tests first: a number you're sure about, a phone call to the target number, a minimal document. They isolate the cause in ten minutes.
• The number format is the leading cause of failure internationally: country code, no leading zero, no stray characters.
• The document must be black and white, 200 dpi, an unprotected PDF, split into batches of 5 to 10 pages.
• VoIP breaks fax: force G.711, enable T.38 if available, drop to 9,600 bit/s, connect directly.
• A number that rings into the void often means the fax machine at the other end is gone — call and ask which channel is now in use.
• Keep the transmission log: it's both the diagnostic tool and the proof of sending.
• For a one-off need, online sending removes the whole hardware layer: send a fax online.