The present document provides a feasibility study for the interworking of multimedia calls between the IP Multimedia CN Subsystem and CS networks (i.e. PSTN, ISDN and GSM/UMTS CS networks). The present document study and outlines different solutions and functionality required within the MGW to deliver the user plane aspects between IM CN subsystems and CS networks for support of basic multimedia calls. It also outlines the solutions and functionality required within the MGCF and SGW to deliver the control plane aspects between IM CN subsystems and CS networks to support basic multimedia calls. The document also studies the MONA interworking impacts, and which parts of MONA are feasible, or even possible, to interwork. The user plane interworking necessary to cover the basic multimedia calls is also outlined and studied. The different aspects studied for the different scenarios should encompass the transport protocol, transcoding and signalling issues for negotiation and mapping of bearer capabilities and QoS information.