Abstract: During call establishment through at least one call server, where at least one calling or called party owns at least one service which is to be invoked each time that a call involving that party is to be established, the following steps are carried out. (a) Any party who owns at least one service which is intended to monitor or control any call involving that party is required to register that service with a call server. (b) Establishing a list of controlling services, and inserting services into the list each time a specific call involving a specific party is being established, by first inserting into the list any service owned by the calling party, and then inserting into the same list any service owned by a called party.