A method of establishing a communication event between an initiating device and a responding device under the control of a remote communications controller, the method comprising implementing by the initiating device the following steps: in a pre-session establishment phase, receiving at the initiating device: a compression dictionary from a dictionary store, or a dictionary link from a dictionary server that identifies an addressable memory location in the dictionary store at which the compression dictionary is held; storing the received compression dictionary or the received dictionary link in electronic storage of the initiating device; generating an initial session establishment request message for transmission to the remote communications controller; applying compression to the initial session establishment request message to reduce its size based on the compression dictionary, by using the stored compression dictionary or by using the stored dictionary link to access the compression dictionary; and 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, establishing a session between the initiating device and the remote communications controller by the initiating device transmitting the compressed initial session establishment message to the remote communications controller; wherein the communication event is established between the initiating device and the responding device based on the established session between the initiating device and the remote communications controller; characterized in that: the compressed initial session establishment request message is transmitted to the remote communications controller according to a preferred transport protocol only if the compressed initial session establishment request message can be encapsulated in a single packet of the preferred transport protocol, the preferred transport protocol being a transport protocol according to which the establishment of the communication event based on said session is initially attempted, wherein the initiating device is configured to otherwise transmit the initial session establishment request message to the remote communications controller according to a non-preferred transport protocol. in a pre-session establishment phase, receiving at the initiating device: a compression dictionary from a dictionary store, or a dictionary link from a dictionary server that identifies an addressable memory location in the dictionary store at which the compression dictionary is held; storing the received compression dictionary or the received dictionary link in electronic storage of the initiating device; generating an initial session establishment request message for transmission to the remote communications controller; applying compression to the initial session establishment request message to reduce its size based on the compression dictionary, by using the stored compression dictionary or by using the stored dictionary link to access the compression dictionary; and 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, establishing a session between the initiating device and the remote communications controller by the initiating device transmitting the compressed initial session establishment message to the remote communications controller; wherein the communication event is established between the initiating device and the responding device based on the established session between the initiating device and the remote communications controller; characterized in that: the compressed initial session establishment request message is transmitted to the remote communications controller according to a preferred transport protocol only if the compressed initial session establishment request message can be encapsulated in a single packet of the preferred transport protocol, the preferred transport protocol being a transport protocol according to which the establishment of the communication event based on said session is initially attempted, wherein the initiating device is configured to otherwise transmit the initial session establishment request message to the remote communications controller according to a non-preferred transport protocol.