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The present paper introduces the concept of network based service triggering in IMS, as well as the functional entity that facilitates this. Network based service triggering ties in with the evolution in IMS interconnect networks and corporate IP networks. New SIP proxy functionality is proposed, Transit CSCF. This Transit CSCF may be realized through enhancement of an existing SIP proxy or may be realized as designated functional entity. The Transit CSCF enables transit SIP or SIP-I based networks to offer services for interconnected networks. One such service may be the connecting of IP-PBXs without requirement for registration of such PBX in the IMS network, but yet with optimized load distribution in IMS network. Transit CSCF leverages on the standardized service assertion methodology of IMS, namely the ISC reference point (of Serving CSCF). With the globalization of IMS networks, IMS interconnect becomes increasingly important for operators. IMS interconnect may be operated by independent parties. The usage of the IMS service control interface (ISC) reference point in transit networks further allows for third party service providers to offer services in these networks.