When setting up Pfsense, the following Firewall->NAT->Outbound manual outbound NAT rule should of been already defined (assumption 1 in the Introduction). This rule translates private addresses in the 192.168.55.xx subnet to the public IP address assigned to the Pfsense WAN interface (and vice-versa).
If the SIP trunk from the ITSP is a static trunk with no registration parameters, then ascertain that the ITSP sends SIP signaling to the public IP address of Pfsense using port 5080 and not port 5060. For both registered and non-registered trunks, Sipxbridge will ping the ITSP address every 20 seconds, as specified in the Devices->SIP Trunk SBCs->sipXbridge-1 Signaling keep-alive interval setting. The Pfsense Diagnostics->Show States command is useful in troubleshooting the firewall states, and which ports are open.
Sometimes an ITSP has two or more 'edge servers' for redundancy and load-sharing, with each edge server having the ability to issue incoming external calls to Sipxcom (e.g.