A method of establishing a communication session in an IMS Centralized Services communication network, the method comprising the following steps: at a Service Centralization and Continuity Application Server: receiving from an originating device a request to establish a session with an Internet Protocol Multimedia Centralized Services, ICS, enabled target device, the request comprising a plurality of codec identifiers; amending the request to include an indication that a circuit switched bearer is to be used between the Service Centralization and Continuity Application Server and the target device; sending the amended request to establish the session from the Service Centralization and Continuity Application Server to the target device, the amended request comprising a plurality of codec identifiers that can be used in the session and an indication that a Circuit Switched bearer is to be used between the Service Centralization and Continuity Application Server and the target device; receiving from an intermediate node between the Service Centralization and Continuity Application Server and the target device an invite message, the invite message including an indication that a codec identifier has been selected from the plurality of codec identifiers at the target device; and sending a message to the intermediate node, the message instructing the intermediate node to establish the session. at a Service Centralization and Continuity Application Server: receiving from an originating device a request to establish a session with an Internet Protocol Multimedia Centralized Services, ICS, enabled target device, the request comprising a plurality of codec identifiers; amending the request to include an indication that a circuit switched bearer is to be used between the Service Centralization and Continuity Application Server and the target device; sending the amended request to establish the session from the Service Centralization and Continuity Application Server to the target device, the amended request comprising a plurality of codec identifiers that can be used in the session and an indication that a Circuit Switched bearer is to be used between the Service Centralization and Continuity Application Server and the target device; receiving from an intermediate node between the Service Centralization and Continuity Application Server and the target device an invite message, the invite message including an indication that a codec identifier has been selected from the plurality of codec identifiers at the target device; and sending a message to the intermediate node, the message instructing the intermediate node to establish the session. receiving from an originating device a request to establish a session with an Internet Protocol Multimedia Centralized Services, ICS, enabled target device, the request comprising a plurality of codec identifiers; amending the request to include an indication that a circuit switched bearer is to be used between the Service Centralization and Continuity Application Server and the target device; sending the amended request to establish the session from the Service Centralization and Continuity Application Server to the target device, the amended request comprising a plurality of codec identifiers that can be used in the session and an indication that a Circuit Switched bearer is to be used between the Service Centralization and Continuity Application Server and the target device; receiving from an intermediate node between the Service Centralization and Continuity Application Server and the target device an invite message, the invite message including an indication that a codec identifier has been selected from the plurality of codec identifiers at the target device; and sending a message to the intermediate node, the message instructing the intermediate node to establish the session.