[Freeswitch-users] Issue with sending fax with t.38

I'm attempting to use txfax in mod_spandsp to send a fax over t.38 to a fax machine at a PSTN number. The receiving side hangs up, after appearing to send a number of malformed RTP packets. "Ignoring invalid RTP packet size of 8 bytes." Can anyone explain this? opensips proxy is 1.2.3.4 fs gw is 1.2.3.5 fs originator is 1.2.3.6 Thanks, James