Abstract: An SIP service method is provided in a network having a NAT. Private address/port of a proxy within the NAT may be registered in a static mapping table of the NAT for accessing the proxy within the NAT from outside the NAT. If messages come to the public IP address/port of the NAT mapped to the private IP address/port of the proxy, all SIP messages may be automatically transmitted to the private IP address/port of the proxy mapped to the public address. If the proxy intends to transmit messages to outside the NAT, a connection is made at the NAT to the outside using the public IP address/port mapped to the private IP address/port of the proxy.