Asterisk 11: T38 Fallback to G711 between Asterisk+Provider


since there is no standard fallback mechanism for T38, we finally encountered a problem with a SIP Provider. Our PBX doesn’t provide T38 yet, so the fallback is inevitable.

Scenario that works without issues:

  1. Sender sends INVITE with G711 in SDP
  2. Authprocess
  3. Receiver is connected
  4. Fax sound
  5. Re-INVITE from Provider with T38 in SDP
  6. Sender sends 488 Not Acceptable here
  7. Provider sends Re-INVITE with G711 in SDP

With the provider we have the following problem:
The provider excepts a Re-INVITE from the sender (PBX) in step 7. Since Asterisk 11 expects the Re-INVITE from the provider the PBX won’t send an INVITE to the provider and the connections times out.

Is there a way to tell Asterisk 11 to send the Re-INVITE instead of waiting for it?


Best regards


The z option of SendFax should cause Asterisk to send the T.38 Re-INVITE anyway. But, you’re talking about ReceiveFax, aren’t you?

Thank you very much.
I’m talking about SendFax.