In this paper we present results, which have obtained by extensive simulations for mobile IP and session initiation protocol from the perspective of VoIP service in wireless Internet access. After illustrating the problem in these two protocols for diverse cases of mobility management, we propose an integrated model, to reduce the handover latency and packet loss during handover. This combination of network and application layer mobility management model reduces disruption time during handovers and provides fast handoff for ongoing conversations. Simulation results presented in this paper are based on the ns2 mobility software. However, since the current version of ns2 does not include SIP model for VoIP service, we add a suite of new features and procedures that are specific to this paper. The simulations results show that our proposed mechanisms achieve better performance than other protocols.