[Serusers] 477 response 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 softwares. 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