apei

I was trying to register a phone device with the Axon 1.2 server, which is running on a win2003 server with two network cards - one connecting to Internet and the other to a private network. The phone has been provisioned with correct user name and password. Its wlan adapter can obtain a private IP address. From the phone I can ping to the Axon server machine's private IP interface. I can also see the SIP registration message on the Axon server machine using network sniffer. However, I did not see any SIP response message generated by the server in the packet dump. The Axon log does not show any SIP response message either. What could go wrong that makes the Axon server not responding to SIP registration messages (targeting UDP port 5060 of the server)? I am using the default setting of the Axon server. Any help will be appreciated.