The public attention on IP telephony and in particular on the SIP protocol , is rapidly increasing. The current circuit-switched GSM mobile network is evolving into the completely packet switched 3rd generation UMTS. The latter provides both voice and data services through an all IP infrastructure. On the other hand, the combination WLAN/UMTS is widely considered since it permits high speed, low cost and large-area coverage. The EVOLUTE IST European Project aims to build an all IP infrastructure that provides multimedia services for 3G users accessing the network through 3G or WLAN technologies. This paper deals in particular with the service creation in this project. A AAA infrastructure based on the triplet (RADIUS, SIP, 3G/AAA gateway) is built to authenticate the service requests and to allow the users to roam in a heterogeneous environment. SIP proxy is used for controlling the media sessions, RADIUS is used to authenticate the services requests and the 3G/AAA gateway translates between the 3G and the IP authentication protocols. The authentication is essentially based on the user's IMSI information. As for the service provisioning, some components such as Presence Server, CPL server and RSTP client with SIP Instant Messaging support were developed. Moreover, an application scenario based on the mentioned framework is fully implemented. It consists of a user booking a video stream service in a 3G domain and visualizing it in a WLAN hot spot. The user is notified about the availability of the service by receiving a SIP Instant Message just after the WLAN registration phase.