According to a first aspect of the present invention there is provided a method of routing a call from an IP Multimedia Subsystem network to a circuit switched network. The method comprises, at a circuit switched carrier select node of the IP Multimedia Subsystem network, receiving a session establishment request (S1), determining if the request includes a circuit switched carrier identifier (S2), and, if not, identifying a circuit switched carrier for the intended destination of the request (S4), identifying a gateway control node responsible for a gateway node connected to the circuit switched network of the identified circuit switched carrier (S5), including in the request a circuit switched carrier identifier for the identified circuit switched carrier (S6), and forwarding the request towards the gateway control node (S8).