Also, there are very good technical reasons why RTP runs over UDP, which actually bear on why RTP was invented in the first place.
With TCP, if you drop a packet, everything has to stop until the recovery mechanism can come into play, but with UDP, a lost or late packet can be interpolated.
TCP would only help with fax if the connection was VoIP from modec to modec, but in that case it would be more efficient to simply send the raw G4 encoded file using a file transfer protocol, or HTTP. If there is an analogue circuit, the retransmissions delays would completely disrupt the timing. Fax data is transmitted synchronously (although it would be possible to buffer lines and add padding).