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One of the benefits of the evolution of the third generation mobile systemss such as UMTS (Universal Mobile Telecommunications System) toward the all-IP network architecture is to provide functionalities to support real-time multimedia services. Simple and efficient multimedia session handling characteristics of the Session Initiation Protocol (SIP), candidates it by 3GPP (Third Generation Partnership Project) for the signalling protocol of IP Multimedia Subsystem (IMS) of UMTS. An important issue in IMS is the fact that service control procedures of the subscribers are performed in the home network. In this paper we review the SIP-based service registration and session establishment procedures in IMS and we study the challenges for providing a SIP-based cooperative mechanism between network operators of different subscribers in IMS in order to prevent extending of service control procedures of roamers to home network and to optimize the session establishment procedure by performing service control procedures of roamers in the visited network.