There's nothing wrong with the wire protocol, but T.38 was designed to work with faxes that don't know it exists. Given that VoIP doesn't provide the same highly-constant latency as the PSTN, T.38 gateways pull some dodgy stuff, such as (if I recall correctly - I no longer have access to this code) deliberately introducing HDLC CRC errors to give themselves time to wait for a packet to appear.