[Serusers] Aastra, SER registration timeout

Hi All, I have Aastra 480i and Cisco both behind a Netscreen firewall. When rebooted the Aastra registers with the SER. The re-registration time is around an hour (it cannot be changed for some reason!). Now after say about 7 mins when I do "serctl ul show" I cannot see the Aastra in the SER at all. The Cisco is still present. Also the Aastra can still make outgoing calls but cannot receive any incoming calls(thats because the SER doesnt have it anymore.) If the NAT bindings are lost, once you make an outgoing call, the phone should receive an incoming call afterwards. But for Aastra something very strange is going on. I cant figure out that why after a short period itself the SER no more has the Aastra in its memory? Does anyone has any clue whats going on? Is there a problem with the phone or SER? Thanks, Hitesh.