I have an SA 540, I also have internal to the network a VOIP enabled phone system. I have NAT rules on this box pointing a public address to the private LAN address on this box. For troubleshooting purposes I have all ports open to the VOIP phone system from this external address. I receive no audio with my external phones, but the phones do register with the phones system. I do have the SIP ALG enabled, I do not have any of the extra features with this box. The PBX is an Samsung Officeserv 7200.
If any has any ideas I would greatly appericate it.