Hi ALex, I am using the publicly available IPtel.org proxy server(195.37.77.99). The ALG is not coming into the picture when a call is initiated from HOST2(external). The SIP packets are not coming to the NAT device when a call is initiated from HOST2. The proxy server itself is sending, " 477 Unfortunately error on sending to next hop occurred(477/TM)" SIP message back to HOST2. So looks like some issue in the Proxy. Could you let me know the possible reason. Thanks Mahesh Mahesh, Most likely problem is that your ALG closes the translation port on inactivity. If you are using cisco pix as an ALG, there is a way to adjust inactivity timeout. I would also suggest getting the most recent version of software. Hope this helps Alex -----Original Message----- From: <http://lists.iptel.org/mailman/listinfo/serusers>serusers-bounces at iptel.org [mailto:serusers-bounces at iptel.org] On Behalf Of mahesh Sent: Tuesday, March 29, 2005 7:21 AM To: <http://lists.iptel.org/mailman/listinfo/serusers>serusers at iptel.org Subject: [Serusers] SIP 477 error when registered with TCP Hi all, I am using the following network configuration. HOST1---NAT---Internet---HOST2 HOST1 and HOST2 are windows XP machines with SJPhone software. The NAT box has SIP ALG(Application Level Gateway), which modifies the SIP packets to replace the Private IP address with the Public IP address. I am registering with iptel.org proxy server using TCP from both the machines. I am able to make a call from HOST1 to HOST2. But when i try to make a call from HOST2 to HOST1, i get an error which says, "477 Unfortunately error on sending to next hop occurred(477/TM)". When I register using UDP, then i do not see this problem. Could someone tell me what I am missing here. Thanks Mahesh