[Freeswitch-dev] Issue when receiving INVITE with both audio and image (T38)

François fdelawarde at wirelessmundi.com
Fri Aug 16 17:56:06 MSD 2013

Hello list,

I've seen several gateways (Patton, Audiocodes, Nortel CS2K), sending
both audio+image (T38) in the initial INVITE. In a simple bridge
scenario, FS will just ignore the T38 part and setup an audio connection
only with b-leg.

My issue is later on if/when we receive a re-INVITE (ex: SIP timers
refresh) with the same SDP (audio+T38), FS now considers it as FAX and
sends a re-INVITE to b-leg with T38 only! In case the destination is
voice only, the call hangs up.


Asterisk apparently had the same issue, they solved it by setting up
both streams on a-leg (RTP + UDPTL). If they receive packets on UDPTL
they just re-INVITE b-leg with T38.

See: https://reviewboard.asterisk.org/r/208/
Also: https://issues.asterisk.org/jira/browse/ASTERISK-11843

Is this the right way to go? (Also is it feasible in FreeSwitch)? I can
add a JIRA if necessary.


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