ReceiveFax to G.711

Home » Asterisk Users » ReceiveFax to G.711
Asterisk Users 3 Comments


While using FFA, how can we program a specific extensions.conf script to
accept the fax in G.711, while for other calls (from a different source), to
allow T.38 with or without fallback?

The current ReceiveFax(filename,f) is not working for us with one of our



3 thoughts on - ReceiveFax to G.711

  • Hi Kevin,

    Controlling it through the sip.conf peers is sufficient for us for this case
    (because this particular provider doesn’t support T.38 at all), but I think
    it would be a good idea to add the option to enable/disable T.38 from the
    dialplan. If I recall correctly, that’s how callweaver worked at the time.

    Also, we just checked it, and since for that provider, we have other codecs
    in higher priorities (like GSM, for example) than G.711, G.711 was not
    chosen as the only codec, so the fax transmission failed. We can not
    prioritize G.711 over the other codecs in the sip.conf, for the obvious
    reasons, so for this, we need to do it in the dialplan. How can we do it?


  • In Asterisk trunk (soon to become Asterisk 1.10), there is an ‘F’ option
    to ReceiveFAX and SendFAX that forces audio mode FAX even if the channel
    is T.38 capable. That would do what you want. I posted a patch some time
    ago for Asterisk 1.8 to add the same ability, but it probably doesn’t
    apply any more… the it’s only a few lines though, it should be fairly
    easy to replicate in the Asterisk 1.8 version of res_fax.c

    How are you going to determine that you need to force G.711 to be used?

  • Thanks. That’s what I did, and it forced G.711, but the problem is that the
    fax failed anyway. Although a lot of RTP packets went through, the end
    result was an empty tif file 🙁

    Kevin, the F option is an excellent idea. Great.