Asterisk FAX via SIP


is it possible to send FAX via Asterisk (incoming/outgoing) over SIP-Interface?
No PSTN card is installed at Asterisk. Most of the topics concerning FAX have to do with PSTN-cards, so I am not sure if it is working.

  • Which drivers/software do I have to install?
  • Are there existing configs (extensions.conf,…)?

Thanks for help!

Gandalf The Grey

the wiki is your friend :smile:

Thanks for the link!


Sending a fax to a SIP device
You can send faxes to SIP extensions, too. Make sure you are using g.711 (Alaw or ulaw) as your codec! Be aware that user reports show mixed results for fax machines connected to ATA devices, for some folks it works, for others it works only 50% of the time. One important aspect is to have an ATA that is able to recognize a fax and immediately turns off its internal echo cancellation routine. Next to that you might have to reduce fax speed from 9600 bit/s to 4800 bit/s in order to produce reliable results. Finally do note that Digium does not claim that Asterisk plays along well with fax transmissions…
Something like this will detect the fax and send it to your SIP device:

exten => s,1,Answer
exten => s,2,Dial(SIP/201&SIP/202)
exten => fax,1,Dial(SIP/203,30)
exten => fax,2,Congestion
exten => fax,102,Congestion

First it answers the line to allow Asterisk to listen for fax tones, then it rings our normal phones. If, however, fax tones were detected, it will go to the “fax” extention which dials our SIP device with the fax machine on it. Finally, if the fax extension is in use or doesn’t answer in 30 seconds, play the congestion tone.

If I use this do I only need “app_rxfax and app_txfax” or do I need another application like Hylafax or Asterfax?