[Serusers] SIP Call termination problem

Hi, I observed a SIP call termination problem with IPTEL proxy server. I am using the following network setup. HOST1---FW/NAT---INTERNET---HOST2 In this setup, SJPhone is used as UA on both HOST1 and HOST2. Now I establish a SIP call from HOST1 to HOST2. The SIP call was established successfully. The call was open for a long time and when i terminate the call on HOST2, i observed that the BYE packet from HOST2 is sent to the proxy server, but the proxy is not forwarding the BYE to HOST1. This behaviour is observed frequently but not always. Does anyone know why the proxy server behaves this way?? Is there any timer maintained by the proxy for the registered UAs if it is behind a FW/NAT device. Regards Mahesh