T.38 with sip authentication

does t.38 with sip authentication supported in asterisk? If it is, how to configure it?

auth => user:password@domain works only for g711 and not on t.38


What are you trying to accomplish?

What is your call path that is involving a request to switch to T.38?

Are you using the spanDSP or Fax For Asterisk backends?


I am using fax for asterisk.

here’s the call flow:

pstn fax----sip provider—t.38—asterisk-----fax

the issue is when asterisk sends the t.38 INVITE to the sip provider it does not contain the sip authentication


If you’re using the paid Fax For Asterisk product, you can get in touch with our support department for assistance via:



sorry, I don’t have support access. I’m just helping out a customer.

The call is coming into Asterisk, right, so Asterisk has already authenticated (if necessary) with the SIP peer, yes?

The original INVITE from the SIP gateway to asterisk is g711 and the SIP gateway does not require authentication for outbound INVITES, then it negotiates a T.38 since the SIP gateway receives a T.38 INVITE from asterisk. When the SIP gateway receives the T.38 INVITE without authentication credentials from asterisk, the SIP gateway responded a SIP 401 which is challenging asterisk to send another T.38 INVITE with sip authentication credentials to be included but asterisk did not respond to the SIP 401 challenge.

BTW, T.38 works successfully without SIP authentication. I wonder if asterisk supports T.38 with SIP authentication.


Dev team thinks it may be a bug.

Can you open an issue on our tracker:




Case ID: QZH-620007