T.38 passthrough error

Having trouble with Asterisk 11.7.0 and T.38 passthrough to a Grandstream HT502 ATA. Both Asterisk and the ATA are behind NAT. The Grandstream is set to receive T.38. SIP trunk carrier is Flowroute.

Some fax messages get through, some don’t. I seem to get the following error no matter what:

[Jan 20 04:09:47] WARNING[980][C-00000022]: chan_sip.c:10353 process_sdp: Failed to initialize UDPTL, declining image stream [Jan 20 04:09:47] WARNING[980][C-00000022]: chan_sip.c:10506 process_sdp: Failing due to no acceptable offer found

Anyone know what this error means?

The remote system isn’t compatible. You will need to provide the complete SDP exchange (use sip set debug on) to find out why.