Most faxes fail because the call drops partway through, nothing answers with a fax tone, or the line is busy; check that the number is really a fax line, send during office hours, and split very long or dark documents. In FaxUp's records, 88.4% of faxes were delivered, and about half of the faxes that hit a problem were rescued by an automatic retry.
A fax is a phone call between two machines, so it fails for phone-call reasons: nobody answers, the line is busy, the call drops. We looked at every fax sent through FaxUp from February 2024 to November 10, 2026 to see which of those actually happen, and what helps.
The numbers
Of 1,666 faxes that reached a final result, 1,473 were delivered (88.4%) and 193 failed after all retries. Retries matter: of the 1,564 faxes with a carrier status log, 288 ran into a busy line, no answer or a failed attempt along the way, and 153 of those were still delivered on a later attempt. More numbers, including how long delivery takes, are in fax delivery statistics. Counts here are totals only, with no personal data.
For the 193 failed faxes, the carrier logs give these reasons. A fax can show more than one across its retries, so the counts overlap:
| What the carrier reported | Failed faxes with this reason | What it usually means |
|---|---|---|
| The call dropped partway through | 59 | The connection broke during the pages |
| Timed out waiting for the first fax signal | 36 | Something answered, but not a fax machine |
| Call failed (no detail given) | 56 | The call didn't connect or ended early |
| Busy | 15 | The fax line was in use |
| Couldn't agree a speed with the other machine | 3 | The two machines' modems couldn't match |
| No answer, other errors | 4 | No pickup, or the machine stopped replying |
Another 27 faxes never got a final report from the carrier and were closed as failed by our own check after 24 hours (with the credit returned), and 42 have no reason recorded.
What each failure means, and what to do
The call dropped partway through
The two machines started talking (the handshake defined in ITU-T T.30) and then lost each other mid-document. It is more likely on long documents, pages with large black areas, and long-distance or noisy lines.
- Send it again later; a different call often gets through.
- Split a long document into two faxes.
- Lighten heavy pages: a dark photo or a scanned page with a grey background takes far longer to send. The fax preview flags dark areas.
No fax signal: something answered, but not a fax
The call connected, but no fax machine answered with fax tones. This is what happens when the number is a voice line, a voicemail box, or a fax machine that is switched off and forwards to voice.
- Check the number against the letter or form. Many offices list phone and fax side by side.
- Call the number from your phone. If a person or a recording answers, ask for the fax number.
- If it's an office, send during their working hours. Abroad, check the time difference; see faxing Australia for an example.
Busy
The line was in use when the call was placed. FaxUp retries automatically, waiting longer each time. In our records, 51 faxes hit a busy line at least once and 36 of them (71%) were delivered on a later attempt.
The machines couldn't agree a speed
Rare: 3 faxes in our records. The receiving machine and the carrier couldn't find a modem speed both support. FaxUp's later retries use standard resolution. If it still fails, the receiving machine may need attention; tell the recipient.
How FaxUp handles a failure for you
- Retries: up to 5 more attempts after the first, 15, 30, 45, 60 and 75 minutes apart. That is close to four hours in total, so a fax that shows "Sending" for a while is often just waiting for its next try.
- Lighter retries: from the second retry, pages go at standard resolution (about half the vertical detail of fine mode), which is quicker to send over a weak line.
- Credit back: if every attempt fails, the fax is marked failed and its credit returns to your balance.
- Refusals before sending: mobile and premium-rate numbers, and countries we can't reach, are refused before dialling, with nothing charged.
A checklist before you resend
- Is it a fax number? Not a mobile, not the main phone line.
- Is the country right in the picker, and the leading 0 dropped for numbers abroad?
- Will the machine be on? Office hours, their time zone.
- Is the document easy to send? No full-page dark photos, no tiny text, and long documents split if they keep dropping.
When it does go through, keep the transmission report as your proof.