Here is my setup:
Cisco UC520 with two imbedded linux platforms attached. One is faxing to the other. Each is running t38modem with opal using SIP.
The CISCO UC520 is acting as a B2BUA and sometimes does drops the first byte of a handshake signal such as MPS. This causes t38modem to respond ERROR to my AT+FRH command which is correct. However, on the next AT+FRH command t38modem responds with CONNECT and then NO CARRIER without a DLE ETX. Subsequent AT+FRH commands are ignored and the call fails.
I am attaching the t38modem trace with +tttt, a wireshark trace, and a trace showing what I am reading and writing.