With ever increasing penetration of IP technologies and the tremendous growth in wireless data traffic, the wireless industry is evolving the mobile core networks towards IP technology. The 3rd Generation Partnership Project (3GPP) is specifying an IP Multimedia Core Network Sub-System (IMS) in UMTS Release 5/6, which is adjunct to the UMTS GPRS CN. This IP-based network will allow mobile operators to provide commonly used Internet applications to wireless user. The UMTS IMS uses the Internet Engineering Task Force (IETF) defined text based Session Initiation Protocol (SIP), to control a wide range of anticipated IP-based services. Text based protocols are easier to develop than bit-wise presentations and likely to be more expediently taken to market, as they allow vendors/operators to open the session/application space to a much broader development community. This is likely to offer new services such as wireless multimedia calls, chat, presence and push services. This paper presents results of a performance evaluation of the SIP based Presence and Instant Messaging service as being standardised by 3GPP and IETF based on a model implemented in a UMTS system simulator called the Framework for Radio Architecture Modeling (FRAM).