HT-503 and fax

I have a Trixbox PBX running 2.6.1.1 and a pair of HT-503s, one for the FXO port to connect to the PSTN, and one for the FXS port to connect to a fax machine. The PSTN line is dedicated for faxes; I use a VoIP service for voice calls. I have verified that I can make calls from the fax FXS extension to other fax machines through the FXO port/PSTN, and receive calls through the FXO port/PSTN to the fax extension. However, I cannot get the fax machines to negotiate. They seem like they are sending the correct tones, but they don't seem like they are hearing each other. I have adjusted the gain on both HT-503s without success.

Does anyone have any suggestions on how to proceed with troubleshooting, or how to configure the T.38 settings so that they negotiate properly?

BTW, to get them to work, I set up the FXS port as a normal Trixbox extension. The FXO port is set up to use a Trixbox trunk. I set up an SIP trunk named HT-503 with the following peer details under outgoing settings:

context=from-pstn
host=dynamic
insecure=very
nat=no
port=5062
qualify=yes
secret=xxxxxx
type=peer
username=HT-503

On the HT-503, make sure the username matches the trunk name and the username in the outgoing settings peer details, make sure the ports match, and under basic settings, Unconditional Call Forward to VOIP, put in @asteriskipaddress:5062, then set up an inbound route for the DID that you just set up, if none already exists.

Thanks.