A communication event is established between an initiating device and a responding device under the control of a remote communications controller. In a pre-session establishment phase: a compression dictionary or a dictionary link that identifies an addressable memory location, at which a compression dictionary is held, is received at the initiating device. The received compression dictionary or the received dictionary link is stored in electronic storage of the initiating device. In response to a communication event establishment instruction received at the initiating device after the dictionary or the dictionary link has been received and stored at the initiating device, a session is established between the initiating device and the communications controller by the initiating device transmitting an initial session establishment message—compressed using the dictionary—to the communications controller to establish the communication event.