Hi there! We have two cisco 7912 ip phone using SIP firmware behind Cisco PIX firewall. These two phone is configured to register to a SER proxy which is on the public Internet. These two Cisco 7912 ip phones are PATor NAPT to a single IP. The problem we have is when these two phones call each other, the two party cannot hear each other's voice. If we replace the PIX firewall with a Cisco router, there is no such problem. Does anyone encounter this problem before? I am clueless on how to resolve this problem. Any pointers will be much appreciated. Thank you very much! Best regards sekchye