The appearance of VoIP technology has now made feasible the use of data network for data, as well as voice communication. In this book, the protocol conversion between two different networks, packet scheduling and routing schemes for gateway selection has been introduced and also verified with Asterisk server. The recent trend for convergence of different access network technologies, market deregulation and emergence of dual-mode end-user devices allow the same destination to be reached by alternative paths and interfaces.Providing a good QoS in heterogeneous networks for irregular traffic flows remains a significant challenge. One of the difficult tasks is that of gateway location, also known as gateway selection, path selection, gateway discovery, and gateway routing. The main objective of this book is to design and develop a PSTN-IP Telephony Gateway (PITG) model and to assist routing packets while ensuring minimal call blocking probability with respect to call arrival rate or load, to increase the overall performance of the system. Specifically, the primary focus is on the development, analysis and optimization of the PITG model and evaluation of a gateway selection algorithm.