I just replied another msg of the same issue on voice system behind firewall. If it’s behind the firewall, you need to have to appropriate ports open for the protocol to pass through. Usually voice system requires tcp ports 1779~1786, and udp ports 10000~20000. If you wanna fine tune the ports, use a port monitor or norton personal firewall software to pick out the exact port. Good luck.
I´m using a Cisco ATA 186 behind a PIX and the only thing I`ve done is give the ATA a public address in “TRANSLATION RULES” and let any access form outside in “ACCESS RULES” (there´s no security issues with that).