In addition to reusing the IMS system as defined by 3GPP, TISPAN is defining an IMS based Multimedia Telephony service, which is an evolution of the CS based Telephony service provided by traditional ISDNs and PSTNs [2]. A similar service, defined by 3GPP, is needed for wireless access to IMS, in order to be able to define services to the end-users, and to support interoperability in multi-vendor and multi-operator environment and to provide the user with the same experience across the different IP based accesses and domains

This Technical Report defines the IMS Multimedia Telephony service and associated supplementary services for IMS. It aims to take account and build on the IMS capabilities already provided in 3GPP Release 5 and Release 6. The objective is to define the minimum set of capabilities required in the IP Multimedia Subsystem to secure multi-vendor and multi-operator inter-operability for the IMS Multimedia Telephony service and related Supplementary Services. Close cooperation shall be sought with TISPAN. TISPAN requirements, as captured in [2], will be taken into account, and referred to where appropriate. While the user experience for the IMS Multimedia Telephony service is expected to have some similarity to existing telephony services, the richer capabilities of IMS will be exploited.