Here’s what worked for me. We were able to solve the 15-minute hangup problem by using the pjsip.endpoint_custom_post.conf file. By adding “timers=no” to the trunk, the calls stopped timing out. I wish there had been a way to configure this in the pjsip trunk settings.
pjsip.endpoint_custom_post.conf
[mytrunk](+)
timers=no
Notice the (+) after the trunk name. It appears to need that to append to the other settings FreePBX manages.