[Serusers] problem for Cisco 7912 ip phone behind pix firewall

[sekchye goh]

Hi there!

   We have two cisco 7912 ip phone using SIP firmware behind PIX
firewall.  These two phone is configured to register to a SER proxy 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