Fax to email on an Asterisk PBX: how it works and why it fails
Fax was designed for analog phone lines, so sending it over the internet needs a few workarounds. Once you understand them, most problems become easy to find.
The basic flow
A fax call comes in over your SIP trunk. The PBX answers it with a fax receiver, turns the received pages into a PDF and emails that PDF to the right person. Sending works the other way round: a PDF goes in, the PBX dials and transmits it.
Two ways to carry fax over SIP
Audio passthrough sends fax tones like a normal call. It works on a clean network but is easily damaged by packet loss and compression. T.38 sends the fax data itself, which is far more reliable, if both your PBX and your provider support it.
Common reasons faxes fail
- The provider does not support T.38, or it is switched off.
- A low-bandwidth codec is squeezing the fax tones.
- Echo cancellation or jitter on the connection.
- Fax speed set too high for the line quality, so lowering it often helps.
- NAT or firewall settings dropping the media stream.
Where to start debugging
Test with a plain G.711 call first, then try T.38, and lower the transmission speed. Check the PBX log for the exact stage where the fax stops.
Telvora PBX has fax to email built in, with a log for every fax. See the features or request a demo.