[Freeswitch-users] T38<>T30 transcoding

Mirko Brankovic mirkobrankovic at gmail.com
Fri Jul 15 18:14:35 MSD 2016

Seems to me that I'm not configuring something correctly or hitting some
Reading https://wiki.freeswitch.org/wiki/Mod_spandsp#t38_gateway

I have a T.38 incapable provider that will respond with SIP 488 back.
If I leave T.38-passtrough and get 488 back, RTP stops and call is dropped.
So I was trying to set up correct transcoding.

This is the scenario, with 3 identical FS:
1 - T.38 incapable server - PROVIDER
2 - T.38 capable - PROXY
3 - T.38 capable - PBX

PBX sends txfax (file.tiff) to PROXY which forwards call to PROVIDER
PBX sends T.38 re-Invite, PROXY responds with 200 OK but then I have 2
scenarios: either passtrough T.38 which will invoke 488 and dropped call or
I tried

sip_execute_on_image=t38_gateway peer nocng"

but this doesn't send anything to PROVIDER but also call is dropped after
BYE and 486 message.

Case that I'm hitting is similar to this:

"If you want to transcode t.38 on leg a to audio on leg B when leg a
invites t.38, but refuse all t.38 offers from leg b "
but to refuse t.38 invites from lag a.

The other way around, call from PROVIDER to PBX over PROXY is easely
transcoded with

sip_execute_on_image=t38_gateway self nocng"

on the PROXY side.

Is it normal behaviour that After failed t.38 negotiation or 488 message,
fax call doesn't fall back to previous codec or t.30?

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