For subscribing a user to a service offered by a first application server (SIP-AS1) of an Internet Protocol Multimedia Subsystem, a request for the service, addressed to a public service identifier of the service, is received from the user in a Call Session Control Function unit (S-CSCF). The request is evaluated based on filter criteria (iCF) associated with the user. In absence of the filter criteria (iCF) associating the service with the first application server (SIP-AS1), the request is forwarded to a second application server (SIP-AS2) based on addressing information obtained from filter criteria (FC) associated with the public service identifier. The second application server (SIP-AS2) transmits to the user a message that includes means for enabling the user to subscribe to the service. Users can be subscribed to a service at the time they desire to use the service, immediately preceding usage of the service. IMS user profiles are used as the basis for deciding whether or not a user is subscribed to a service, consequently, there is no need to provide interfaces to data repositories of multimedia services for determining the subscription status of a user. Subscription mechanisms for multimedia services can be centralized in a dedicated application server.