Got this working on a single-WAN pfSense this past week, quick rundown of my pfSense settings:
NAT forward TCP/UDP ports 10,000-20,000 to the internal IP of the IPiphony, and NAT forward port 5060 to the same internal IP.
Go to Firewall -> NAT -> Outbound and set it to Manual Outbound Rule Generation. Edit the "Auto created rule for LAN to WAN" and make certain the Static Port box is selected. Save and reboot then feel the joy!
Anybody get this working with multiwan failover? My failover works but we have to reset the state table or (easier for the end user) reboot the pfSense computer to bring the VOIP system back up.
The system had been having issues with disconnects on forward and I'm not certain if they're with the VoIP provider, the PBX, or the pfSense firewall. The vendor who is in charge of the PBX tested it with a Cisco firewall and found the behavior continued, but I'm still not certain as its possible the Cisco device is subject to the same failure.
If the undesired behavior continues I will be running through the troubleshooting steps at http://doc.pfsense.org/index.php/VoIP_Configuration.