A method of establishing a telecommunications session in a network having IP Multimedia Subsystem (IMS) and circuit switched (CS) domains between a first terminal registered with the network and a second terminal is disclosed. The method includes the steps of receiving at the network a message from the second terminal in the CS domain for initiating a session with the first terminal; determining at the network whether the first terminal is available to establish an IP Multimedia Subsystem session; and, establishing an IP Multimedia Subsystem session or a CS session between the first terminal and the second terminal in dependence thereon.
