By David Thompson · Published November 22, 2024 · Updated June 8, 2026 · 8 min read
Quick Answer: The most common reason a fax won't go through is a wrong number, a busy recipient line, or a VoIP phone line that corrupts the signal. Check the number, wait 5 minutes, and retry. If it still fails, try the 8 fixes below — or skip the machine entirely and send via mFax.to.
Approximately 5% of traditional fax transmissions fail to reach their destination. On VoIP lines, that number jumps to 5-20%. If your fax isn't going through, you're not alone — and in most cases, the fix takes under five minutes.
This guide covers the 8 most common reasons faxes fail and the exact steps to fix each one. Whether you're using a standalone fax machine, a multifunction printer, or an online fax service, start at Fix #1 and work down until your fax goes through.
In a hurry?
If you need this fax delivered right now and can't spend time troubleshooting hardware, skip to the instant alternative at the bottom. Online fax services bypass every physical-line issue on this list.
Quick Diagnostic: What Error Are You Seeing?
Before diving into fixes, check the error message on your fax machine's display or confirmation report. Here's what the most common errors mean:
| Error Message | What It Means | Jump To |
|---|---|---|
| Busy / No Answer | Recipient line occupied or machine off | Fix #2 |
| Communication Error | Connection dropped mid-transmission | Fix #4 |
| No Fax Detected | Recipient number is a voice line, not a fax line | Fix #1 |
| NG (No Good) | Generic transmission failure | Fix #4 |
| Negotiation Failed | Machines can't agree on speed/settings | Fix #5 |
| ECM Error | Error Correction Mode conflict | Fix #6 |
If you don't see an error message at all, your fax machine should print a confirmation page after every send attempt — check the paper tray.
The 8 Fixes
Wrong or Invalid Fax Number
Cause: Transposed digits, missing area code, or dialing a voice line instead of a fax line.
This is the single most common reason faxes fail. Even one wrong digit sends your document to a stranger's machine — or to a voice line that simply rings and hangs up.
How to fix it:
- Verify the number directly with the recipient. Don't rely on old contact cards or websites.
- Include the full area code, even for local numbers.
- For international faxes, dial the exit code + country code + number. See our fax country codes guide for the correct format.
- If the machine says "No Fax Detected," the number is likely a regular phone line, not a fax line.
Recipient Line Is Busy or Not Answering
Cause: The receiving fax machine is in use, out of paper, turned off, or not set to auto-answer.
A busy signal means someone else is sending or receiving a fax on that line right now. "No Answer" means the machine is off, disconnected, or the ring count expired before it picked up.
How to fix it:
- Wait 5 minutes and retry. Most fax machines have an automatic retry setting (typically 3-6 attempts at 5-minute intervals).
- If the line is consistently busy, call the recipient and ask them to clear the line or confirm the machine is on.
- After 4 failed attempts, stop retrying the same way — the problem is likely something else on this list.
VoIP Phone Line Is Corrupting the Signal
Cause: VoIP compresses audio signals, destroying the analog tones fax machines need.
This is the fastest-growing cause of fax failures. If you recently switched from a traditional phone line to VoIP (Vonage, RingCentral, Ooma, or an office IP phone system), expect significantly higher failure rates. VoIP codecs like G.729 compress audio and strip out the frequencies fax machines depend on.
How to fix it:
- Switch your fax line to the G.711 codec (uncompressed audio). Call your VoIP provider and ask them to enable it for the port your fax machine uses.
- Ask your provider to enable T.38 fax protocol — it's specifically designed for fax over IP and is far more reliable than audio passthrough.
- Disable any silence suppression, echo cancellation, or noise reduction on the fax port — these features interfere with fax tones.
- Lower the baud rate to 9600 or lower (see Fix #5).
For a full technical deep-dive, read our guide to VoIP fax problems.
VoIP Fax Failure Rates
Fax over unoptimized VoIP connections fails 5-20% of the time — compared to ~5% on traditional analog lines. Even with T.38 and G.711, VoIP fax is less reliable than cloud-based fax services that bypass VoIP entirely.
Poor Phone Line Quality
Cause: Line noise, static, or interference causes the connection to drop mid-transmission.
Fax machines are sensitive to line quality. Even minor static — imperceptible during a voice call — can corrupt fax data and cause a "Communication Error" or "NG" (No Good) result.
How to fix it:
- Pick up a phone handset on the same line and listen. If you hear static, crackling, or humming, the line has a quality issue. Contact your phone provider.
- Check all physical connections: wall jack, phone cable, and the fax machine's LINE port. Replace any damaged cables.
- If other devices share the line (answering machine, DSL modem), temporarily disconnect them and retry.
- For long documents (10+ pages), try sending in smaller batches — longer transmissions are more vulnerable to line dropouts.
Baud Rate Too High
Cause: The sending fax machine is trying to transmit faster than the phone line can handle.
Fax machines negotiate a transmission speed (baud rate) during the initial handshake. If the agreed speed is too high for the line quality, the transmission will fail mid-page. This is especially common on VoIP lines and older infrastructure.
How to fix it:
- Access your fax machine's settings and lower the baud rate from the default (usually 14400 bps) to 9600 bps.
- On VoIP lines, try 4800 bps if 9600 still fails.
- The setting is usually found under Fax Settings > Transmission Speed or Communication Settings > Max Baud Rate.
- After sending successfully, you can try increasing the speed again — some lines handle 9600 fine but choke at 14400.
Error Correction Mode (ECM) Conflict
Cause: ECM is causing retransmission loops on a noisy or VoIP line.
Error Correction Mode (ECM) detects corrupted data during transmission and automatically requests the sender to resend damaged sections. On a clean analog line, this is helpful. On a noisy or VoIP line, ECM creates an endless loop of retransmission requests that eventually time out.
How to fix it:
- If you're on a VoIP line, turn ECM off. The retransmission attempts cause more problems than they solve on compressed connections.
- If you're on a clean analog line, leave ECM on — it improves reliability.
- The setting is usually under Fax Settings > ECM or Error Correction Mode > On/Off.
ECM Rule of Thumb
Analog line = ECM on. VoIP line = ECM off. If you're not sure what type of line you have, check our guide on analog vs. digital fax.
Another Device Is Answering Before the Fax Machine
Cause: Voicemail, an answering machine, or call waiting picks up the line before the fax machine can respond.
If your fax machine shares a phone line with voicemail, an answering machine, or a phone with call waiting, those services may intercept incoming fax calls or disrupt outgoing ones with call waiting tones mid-transmission.
How to fix it:
- Disable call waiting before sending. On most US lines, dial
*70before the fax number to temporarily turn it off. - If an answering machine shares the line, set the fax machine to pick up on fewer rings than the answering machine (e.g., fax on 2 rings, answering machine on 4).
- Disconnect the answering machine entirely when sending important faxes.
- Ideally, use a dedicated phone line for the fax machine — no voicemail, no call waiting, no shared devices.
Document or File Issues
Cause: Corrupted files, password-protected PDFs, or unsupported formats (online fax services), or dirty scanner glass and paper jams (physical machines).
On a physical fax machine, a dirty scanner glass, worn rollers, or a paper jam can cause the machine to send a blank page or abort mid-send. On online fax services, password-protected PDFs, corrupted files, or unsupported formats cause the send to fail before it even reaches the phone network.
How to fix it:
For physical fax machines:
- Clean the scanner glass with a lint-free cloth and glass cleaner.
- Check for paper jams in the automatic document feeder (ADF).
- Run a self-test print to verify the machine is scanning correctly.
For online fax services:
- Use PDF format — it's the most universally supported.
- Remove password protection from PDFs before uploading.
- Keep documents under 50 pages per send — very long faxes are more likely to fail.
- If you converted from Word or another format, open the PDF first to make sure it rendered correctly.
Fax Sent Successfully but Recipient Says They Didn't Get It
This is a frustrating scenario: your confirmation page says "OK" but the recipient claims nothing arrived. The fax did transmit — the problem is on the receiving end.
Common causes:
- Recipient's machine is out of paper or toner. The fax data was received but couldn't be printed. Newer machines store it in memory; older ones discard it.
- Fax went to the wrong machine. If the recipient has multiple fax lines or their number was recently reassigned, your fax may have landed on a different device.
- Recipient didn't check. Physical faxes sit in a paper tray. If multiple people share the machine, your fax may have been picked up by someone else.
- Memory full. The receiving machine's memory was full and it silently dropped the transmission after acknowledging receipt.
What to do: Call the recipient, confirm the fax number, and ask them to check the machine's paper tray, toner, and memory status. Then resend.
When to Stop Retrying and Try Something Different
If you've worked through the fixes above and the fax still isn't going through after 4-5 attempts, the problem may be outside your control — the recipient's machine is down, their phone line has issues, or there's a carrier-level outage.
Before you give up, try a test fax to a known-working number. If the test fax goes through, the problem is on the recipient's end. If the test also fails, the problem is your line or machine.
Skip the Troubleshooting: Fax Online Instead
Every fix in this guide addresses problems inherent to physical fax machines and phone lines — busy signals, VoIP interference, line noise, hardware errors. Online fax services bypass all of these issues.
When you send a fax through mFax.to, here's what happens differently:
- Your document goes from your phone or computer to a dedicated fax gateway over the internet — no phone line on your end
- The fax gateway connects to the recipient's fax machine via the traditional phone network using optimized, carrier-grade infrastructure
- You get a delivery receipt by email or push notification — no guessing, no confirmation page to dig out of a paper tray
No VoIP codec issues. No busy signals on your end. No dirty scanner glass. No phone line to troubleshoot.
98% Delivery Rate
mFax.to delivers faxes with a 98% success rate — compared to ~5% failure on traditional analog lines and 5-20% on VoIP. Upload a PDF, enter the number, and send from any device.
If you're regularly troubleshooting fax failures, the fastest permanent fix is switching to an online fax service. See our comparison of the best online fax services for a detailed breakdown, or read our complete guide to sending a fax for all available methods.
Send Your Fax Now
If the fixes above solved your problem, great — your fax should be on its way. If you're still stuck, skip the hardware entirely.
Open mFax.to, upload your document, enter the recipient's fax number, and send. It works from any phone, tablet, or computer — no fax machine required.
For business teams that fax regularly, mFax Business offers virtual fax numbers, delivery tracking, and HIPAA-compliant infrastructure starting at about $9/mo (billed annually). There are no rigid fixed tiers — you build your own plan, choosing the exact seats and monthly pages you need with a live calculator and paying only for what you use.