In SIP network environment, a general NAT traversal method will become invalid when an NAT with ICMP (Internet Control Message Protocol) is met. The present invention provides four sessions for SIP, i.e. Login Session, Port Prediction Session, Synchronization Session and Media Session, and the SIP network environment includes a first Internet telephone, a second Internet telephone, a first symmetric NAT, a second symmetric NAT and an SIP proxy server. The first symmetric NAT and the second symmetric NAT are ICMP-sensitive. In the Synchronization Session, the first Internet telephone and the second Internet telephone are designed to transmit packets synchronously to avoid port locking.