A method of enabling configuration of a link between a circuit-switched Private Branch Exchange (CS PBX), and an access gateway node that connects at least one CS PBX to an IP Multimedia Subsystem (IMS). The method is performed at an IMS application server, AS. The AS is configured with timeslot configuration information for each link between the access gateway node and the at least one CS PBX. The AS receives, from the access gateway node, a request for timeslot configuration information for one or more of the links, and identifies the one or more links to which the request relates. The AS then retrieves the timeslot configuration information for the identified link, and sends a response indicating this information to the access gateway node.