The subject of this thesis is interworking between circuit switched mobile networks and IP Multimedia Subsystem (IMS) and particular demands on Session Initiation Protocol (SIP) due to unification of services in converged network. Special attention is put on the call forwarding services triggered in the IMS domain for calls with the source in the mobile network. As a solution for the convergence of fixed and mobile networks and convergence of services, IMS imposes requirements for the interaction of the existing circuit switching networks and the IMS. Media Gateway Controller Function (MGCF) as SIP endpoint is responsible for the interaction of SIP protocol used in IMS network and ISUP or BICC protocols used by circuit switching network. Providing additional services in the IMS network, such as PSTN simulation services, puts additional demands on the SIP protocol. Service requiring additional support by the SIP protocol and MGCF which is explored in this paper is a call forwarding service. This thesis describes a model to establish forwarding services in the IMS network to the level of SIP session. Analysis of processes and functions in the IMS network leads towards requirements on MGCF node as a control point of contact for calls from the source in the mobile switching network. The thesis explains the basic architecture of MGCF and SIP protocol in the combined MSCS/ MGCF node with Mobile Switching Center function in layered architecture (MSC-S) and MGCF, developed by Ericsson. Overview of MGCF and SIP updates necessary for call forwarding service interwork is explained. The results of the research of call forwarding services that is triggered in the IMS network are test cases used for verification of different scenarios with the Call Forwarding on No Reply (CFNR) service for a call from a mobile network. Test cases are used for verification of MSCS/ MGCF SIP software. Presented results confirm the correctness of solution and shows no faults in implementation. Testing and verification is carried out under laboratory conditions using simulators of actual MSC-S/MGCF node.