Telecommunications services like instant messaging or internet telephony bring increased flexibility to communicate at home, in the office or on the move. Their pervasiveness is also a source of disruptions and intrusions. Service providers are therefore looking for personalisation solutions allowing users to control the timing and modalities of their communications. In the case of telephony services, personalisation solutions are built around call control features. Technically, a feature is an increment of functionality that modifies the basic system behaviour. Dozens of call control features have been created to address concerns such as mobility, privacy, presentation, or billing (see Figure 1).

Figure 1: Feature and Feature Types
Features are designed as independent separate module and are exposed through feature catalogs (see Figure 2). A user selects from the catalog the set of features that he wants to be active during the session in which he is involved. He may also want to sequence the activation of the selected features in a particular way to better suit his needs (e.g., he may want to filter his incoming calls before diverting them). The features that the user has selected plus the ordering constraints that he has imposed on them constitute the feature subscription of the user.

Figure 2: An Example of a Catalog
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