Assuming that you want FusionPBX to register to FreePBX (as if FreePBX is the trunking provider), try these settings:
Trunk Name: (matches username in FusionPBX)
In PJSIP Settings → General:
Authentication: Both
Registration: Receive
Secret: (matches password in FusionPBX)
Context: from-internal
(Username, Auth username, SIP Server and SIP Server Port are greyed out and can be left blank)
In PJSIP Settings → Advanced:
Match Inbound Authentication: Auth Username
Rewrite Contact: Yes
If you have trouble, paste the Asterisk log (with pjsip logger on) at pastebin.com and post the link here. If the remote PBX fails to register, paste the log for a registration attempt. If registration is ok, paste the log for a failing incoming and/or outgoing call.
