First, check that in Asterisk SIP Settings, External Address and Local Networks are correctly set. If you change these, after Submit and Apply Config you must restart Asterisk.
Next, capture traffic on pfSense WAN interface and see whether the incoming INVITE appears. If yes, find out why the network configuration is not passing it to the PBX.
If no, look at the registration request in sngrep to see whether the Contact header shows correct IP and port. If yes, ask for a SIP trace from the provider; your ISP may be blocking ports or mangling the traffic.