The integration of IP-based and Cellular network has lead to increased demand on internet applications. Wi-Fi handsets that are capable of Voice over Internet Protocol (VoIP) services and that can switch seamlessly between IP-based and Cellular networks are subjected to continuous security threats that prompt the need for security mechanism. In this paper a solution for a secured VoIP service in this heterogeneous environment is proposed. Specification of different technical security tools has been highlighted on which multilayered security tools are proposed. The framework will serve as the security reference guide to Mobile Network Operator (MNO) as they integrate with Wi-Fi network systems in order to provide VoIP services.