Nevertheless, to deploy the IMS overlay, WiFi operators may choose a step by step and evolutionary strategy to avoid a sudden huge cost for deploying the IMS services. To this end we have proposed several phases for adopting IMS in WiFi technology. In the first phase, as depicted in Fig.6, the WiFi operators may just implement the functionality of P-CSCF. The P-CSCF will define the security strategy and the compressing protocol to reduce the size of SIP messages. For inter operability with IPv6 domain, NAPT (Network Address/Port Translator) and ALG-Application Level Gateway functionalities are required. NAPT should be implemented in edge router to replace the IPv4 and IPv6 addresses at the edge of the network in bearer level.