In the next generation networks, the users should have access to unified services seamlessly via any kind of network technology. Overlay approaches with unified signalling protocol are considered as the major solutions to realize the convergence of communication services. This book concentrates on two main topics: i) Extension of IMS to WiFi and converged communication services in 3G/WiFi and ii) Design of P2P Telephony overlay compatible with IMS/SIP on ad hoc networks. Vast deployment of WiFi technology in the scale of a city to provide pervasive internet access is happening more and more. Hence, providing a concrete IP multimedia telephony service system based on this technology provides cost-effective mixed Telephony/Internet services convergent to the 3G service domain. Subsequently, MANET is going to play a significant role in future network technology. Deployment of a P2P IP telephony system based on SIP is a potential solution which from one side satisfies the required auto-organization feature and from the other side keeps compatibility and inter- operability with the existing SIP based platforms such as IMS.