Delete all the rules above and create a port forwarding rule:
Everything that hits the external interface's IP on port 5060 is forwarded to the PBX on 5060.
This should give you the main connection. Then check the udp port range the PBX uses for actual communication (RTP).
Forward those ports as well from external IP to the PBX.
If the RTP ports cannot be nailed down to reside in a certain range, check if the PBX can use a STUN server and if your provider offers one. If so, the PBX connects to the STUN server, does a handshake when it comes to ports and then uses those ports on the firewall (punches holes in the state table for said udp ports) and keeps them open and alive.