IP Multimedia Subsystem (IMS) is envisioned to be the common platform for service provisioning in heterogenous networks. A variation of the Session Initiation Protocol (SIP), is proposed for session control in cellular networks based on the IMS architecture. SIP is also used in many corporate networks with 802.11 wireless access. The interconnection of these two SIP-based domains for extended coverage and persistent service is one of the promising steps for heterogeneous network convergence. In this scenario, service access is performed through two different wireless access networks. A solution for migrating existing sessions between these two domains, known as vertical handover is proposed in this thesis. The solution is based on modifications of standard SIP signaling and allows full mobility between corporate and cellular domains. As part of the study, the client requirements from a SIP perspective are also analyzed and a solution in the form of states machine is proposed. Results show that it is feasible to implement a soft-handover with the support of SIP for session establishment at the IP level.