I am trying to interface to a Avaya PBX and TCP is required for inbound/outbound transport protocols. I have the inbound calls working with TCP but according to wireshark all outbound communications from asterisk/freepbx is UDP.
Asterisk 1.8.15 certified
not sure about freepbx, it was recently downloaded
Depending on your version of FreePBX, which you don’t know, (but should if you post here 
), current versions will allow you to set the “transport” in your sip extensions to use tcp,udp,tls or various combinations, Did you you RTFM yet?
RTFM?
I can change transport options in the extensions but that has not effect. A extension set to TCP only will still make a UDP packet out of Asterisk.
the FM will let you know that you need to also:-
tcpenable=yes
tcpbindaddr=(your network space that it will bind to)
in your /etc/asterisk/sip* heirarchy,
Perhaps the easiest way in newer versions of FreePBX is to do it in “Other SIP Settings”