Good to know
Faxing over VoIP, especially outbound, is known to be tricky, particularly when using an ATA (Analogue Telephone Adapter). Even though our systems fully support T.38 fax protocol, many issues arise from how well (or not) the fax machine and ATA work together.
This is because the fax standard isn’t consistently followed across devices. The ATA, the fax machine, and their individual settings all play a part in success or failure.
Inbound faxing is fully handled by us:
If you use a fax line from us, all inbound faxes are processed directly on our system and sent to you as PDFs. You don’t need to worry about receiving faxes.
Setting up your ATA and Fax Machine
You can use the same SIP account details you’d use for a normal VoIP phone setup with us, we keep it simple on our end.
But setting the ATA to work correctly with a fax machine will require adjusting other settings. These vary depending on the manufacturer, device model, and even firmware version.
Please follow the setup guide from your ATA and fax machine’s manufacturer for their part of the configuration.
Troubleshooting Guide
Your device’s settings might not match these options exactly, but the terms are often similar. Here are the key things to try:
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Transmission speed:
Start with 9600 baud
If it works well, you can try increasing to 14,400, but that may cause more problems if your network isn’t stable. -
Codec:
Set to G711u -
Preferred Codec Only:
Set to Yes -
FAX Passthru Method:
Set to NSE (if available)
If not, protocol-based passthrough is fine -
Port type:
Try UDP first
If needed, test with TCP, always on port 5060 -
Call Waiting:
Turn Off on both the ATA and the fax machine -
Silence Suppression:
Turn Off -
Echo Suppression:
Turn Off -
Echo Canceller (ECAN):
Turn Off -
Error Correction (ECM):
Turn Off -
Overseas Mode:
Turn On (if available) -
Jitter control:
Try switching it on and off
If on, test different values-
Larger values = more stability, slower transmission
-
Smaller values = faster, but more fragile
Find what works best for your line
-
-
SIP ALG:
Disable it completely
This may require using the command line instead of the web interface -
TX Volume:
Adjust as needed, incorrect levels can cause failures
Optional tweaks if issues remain
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Try different settings for TXgain and RXgain (if available in your FXS settings)
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Experiment with Calling Channel Codec Priority (if available)
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Test if enabling QoS (Quality of Service) helps
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Best set on the modem/router
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Alternatively, apply on the ATA if supported
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Still having trouble?
We can check our log files to help diagnose the issue. But we’ll need details from the specific failed fax attempt, no more than 12–24 hours old.
When your next outbound fax fails, please send us:
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The number you faxed to
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The time of the call (as exact as possible)
The more recent the call, the easier it is for us to find and review the relevant logs.
Recommendations
Over the years, we’ve seen the good, the bad, and the downright ugly when it comes to faxing over VoIP.
Some setups work straight away and keep working reliably. Others never work at all, no matter what you try.
That said, in our experience (and please don’t hold us to this as results may still vary), one combination seems to stand out when it comes to ease of setup and reliability:
✔️ A Linksys ATA, such as the Linksys PAP or PAP2T
✔️ Paired with an HP fax machine
Even then, results depend heavily on the model, and strangely enough, the simpler machines often seem to work better than the more advanced ones.
Last updated: 01/04/2025
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