—As wireless networks evolves towards the 3G and 4G architectures the mobile core networks provides a platform for all-IP convergence of mobile and fixed networks allowing for better integration of the Internet and cellular networks, providing better quality of service, charging mechanisms and integration of different services. Due to this convergence, there have been various evidences of interoperability limitations between the mobile and fixed networks such as protocol adaptation, end-to-end Quality of Service (QoS), charging and signalling interworking. Most of these limitations are currently been addressed by respective standards bodies and various proposals are being implemented. In this paper we investigate the signalling limitations across converged networks which are caused by the use of different Session Initiation Protocol (SIP) profiles for signalling in both mobile and fixed networks. We carried out various performance based simulations on the 3GPP proposed solution in wh...