It is for sure that VoIP is the key technology of the next generation network to offer real-time multimedia communication. VoIP provides the communication community with a totally new solution to create and integrate new services across different networks. SIP, among others, is the most popular protocol in VoIP community now. In order to keep its popularity, SIP must solve some ViIP technology issues for interworking with PSTN. This paper depicts the necessary mechanisms for interworking. There are two versions of SIP-based protocols used to interwork VoIP with PSTN. One is SIP-T, issued by IETF. The other is ITU-T Q.1812.5, published by ITU-T. They are very similar aside from some parameters’ mapping methods. This paper describes these mechanisms based on SIP-T, firstly, and then illustrates the architectures defined at Q.1912.5. Finally, some differences between them are listed for comparison.