The Session Initiation Protocol (SIP) is likely to play a key role in the convergence of Internet and the conventional cellular networks. Several issues related to mobility management in SIP enabled networks remain to be resolved. While other work has compared and contrasted mobility management in SIP and Mobile IP (MIP), in this paper we investigate possible integration techniques that combine the mobility management capabilities of both protocols. After illustrating some of the issues with each protocol when used in isolation in various mobile VoIP scenarios, we propose an integrated model (MIP+SIP), that reduces the disruption time during handovers. Our combination of network and application layer mobility management models reduces the global signaling load and provides fast handoff for ongoing conversations. Simulation results show that our proposed mechanisms achieve better performance than isolated SIP and MIP mobility management. Simulation results presented in this paper are based on the ns2 mobility package[6], which we extended with new capabilities to model SIP components (i.e., user agents, redirect servers, proxy servers, registrars) and SIP-based VoIP traffic.