I’m having problems faxing if the B leg offers T.38.  Several scenarios, but always the same result.

First, the architecture

FAX à ATA (T.38 optional) à FS à Carrier  à Destination (Possibly another carrier (PSTN) and the fax machine)

On my ATA, T.38 is enabled (Auto Detect).

If the destination supports T.38, the Carrier will offer it to Freeswitch. If the Destination doesn’t support T.38, the B leg doesn’t offer it and the faxes go through.

I’ve set Freeswitch as:

      <action application="set" data="fax_enable_t38=true"/>

      <action application="export" data="t38_passthru=true"/> 

When that’s the case, and the B leg offers T.38, Freeswitch passes through the offer and the ATA responds, 3 times. But the response never gets sent to the B leg, so it terminates the call after about 30 seconds.

When I set Freeswitch as:

      <action application="set" data="fax_enable_t38=false"/>

The B leg still offers T.38 if its other side is capable. Freeswitch doesn’t pass the offer through and the B leg terminates the call after about 30 seconds.

I’m running on a recent git ( FreeSWITCH Version 1.0.head (git-86d757d 2011-05-17 22-51-47 -0500) ).

I’ve had no success attempting to configure FS as a gateway. Frankly, T.38 “passthrough” has had the most success, mostly when the other ends aren’t T.38 capable and it stays in G711u.

Is there something else I need to do to enable T.38 in passthrough mode? 

Thank you,
Rob Morin

PS – I can provide tcpdumps of this, or whatever else is necessary.