defined as a pragmatic selection of existing standards related to Session Initiation Protocol with the selection of audio, video and text codecs. It is a private initiative of services provider and technology vendors seeking interoperability in the broadtest sense. Secondly, this can apply to existing and deployed SIP platform rather than future or next generation networks. Thirdly, IMS is much more ambitious and define a whole architecture, including all internal interfaces and billing and physical infrastructure. These standards are more suitable for large telecommunication operators and their equipment vendors. This complexity has consequences: Very little UMTS release 5 networks fully supporting IMS are in production Multimedia call services defined by MMTEL are not translated in actual usable services on the market. An industry led initiative called Rich Communication Suite finally plans to do this .. when IMS will be rolled out in the networks. Rolling out a Native SIP Total Conversation services needs investments that are very reduced compared to the IMS equivalent. However, the implementations in both environments are close enough that interoperability between the two is an easy task. References ITU-T recommendations Accessibility