@wintok Hi wintok,
I have another option for you that may assist you with overcoming your problem.
I have just noticed that FreePBX has OpenVPN available.
pfSense also has OpenVPN available.
Setup a VPN tunnel between FreePBX and pfsense then let your phones communcate with FreePBX over the VPN tunnel. It should then be a matter of traffic rules to get it all working.
Here is a link with some instructions.
https://wiki.freepbx.org/display/FDT/%5BHow-to%5D+Setup+VPN+between+pfsense+and+FreePBX
Cheers