At present I don't see any feasible way of doing this at all with FreePBX. You can create a transport easily enough, by manually adding the necessary details to
pjsip.transports_custom.conf, and you can confirm that the transport is created by the Asterisk output from
pjsip show transports. But you can't apply this transport to a PJSIP trunk (nor to an extension) directly from the GUI, so you must also manually create a PJSIP trunk then manually create dialplan to use the trunk. At that point, using the GUI you would create a trunk of type 'Custom' with an appropriate dial string that matches your dialplan.
This needs a feature request to allow users to create custom transports in Asterisk SIP settings.
edit - Having finished coffee #3, I realize there is a feasible way to do this. Create a new PJSIP transport in
From the Asterisk CLI confirm it is working:
pjsip show transport TESTING
In the file
pjsip.endpoint.conf locate the endpoint associated with the trunk you want to change, and note the name in square brackets. Then create something like the following in
pjsip.endpoint_custom_post.conf using the name that matches your trunk:
.Then check the transport line from the Asterisk CLI:
pjsip show endpoint pjsip-test-trunk