[Serusers] Proxy sending ACK on port 5060 instead of on contact port

Hi all, I am using the following configuration to test SJPhone through IPTEL proxy server(publicly available:195.37.77.99). HOST1(win2K)---FW1---Internet---FW2---HOST2(winXP) Host1 and Host2 are hosts with private IPs having SJPhone application. FW1 and FW2 are firewall/NAT devices with SIP-ALG implementation. This means all the SIP messages are modified to include the public IPs in them. Also the via,contact ports are modified. Now both host1 and host2 register with the iptel proxy server. A call is initiated from host1. Host2 sends 200 OK response to the HOST1. After this the ACK sent by Host1 is sent to port 5060 on HOst2,instead of the contact port advertised in 200 OK. This is observed from ethereal captures on Host2. All these messages are going through the proxy server(195.37.77.99). Can someone please tell me why the proxy server is behaving this way, i.e. sending ACK on 5060 instead of the contact. Is there anything that i am missing here or doing wrong. Thanks Mahesh