Hi All, To be more specific on the problem, I am using the following network configuration. HOST1 XP---FW/NAT---Internet---HOST2 XP Network Configuraton: Host1's IP address : 172.16.4.8 FW Internal address: 172.16.4.1 FW external address: 202.125.84.163 Host2's IP address : 202.125.84.164 SER(public server) : 195.37.77.99 FW/NAT has SIP-ALG implementation. SJPhone is installed on both Host1 and Host2. Now Host1 registers with the SIP proxy server. The register request after the NAT(ALG) process has the src port of 55001(IP:202.125.84.163) and the contact port of 60002(IP:202.125.84.163). Similarly SJphone on Host2 is also registered with the proxy server. Now when a call is made to Host1 from Host2 using SJPhone, the SIP invite request coming from Host2 is sent to the proxy server, which in turn is sending it to 202.125.84.163 on port 55001(source port of register). This request is supposed to come on port 60002(contact port of register). A month back when i tested in the same configuration, the request from the external machine was coming on the contact port of the register request(according to the RFC 3261). But when i tested it yesterday it was having the mentioned problem behavior. Could someone please let me know if anything is changed in the proxy server(SER) which is causing this behavior. Also how this behavior can be avoided. Thanks in advance. Best Regards Mahesh