Initially, as shown in step s400, the mobile telephones 3-1 and 3-2 establish a device-to-device communication path, using their respective D2D control modules 45, in order to communicate with each other. At this phase, the mobile telephones 3 (or at least one of them) are located outside the coverage area of the cellular network. In step s401, the mobile telephones 3 start using the D2D service for voice and/or data communication between them.

After initiation of the D2D service, as shown in step s402, the D2D control modules 45 coordinate the mobile telephones' D2D transmission such that there are transmission gaps (e.g. ‘silent’ periods) in the D2D communication when the mobile telephones 3-1, 3-2 do not transmit data to provide opportunities for the mobile telephones 3-1, 3-2 to monitor for the presence of the cellular communication network and perform measurements. This step also comprises exchanging measurement gap pattern(s) between the mobile telephones to ensure appropriate synchronisation.

In step s403, the mobile telephones 3 begin to search for a cellular network using their cellular network monitoring module 46. In this example, the cellular network monitoring module 46 uses a similar procedure that is used by RRC Idle mode mobile devices when selecting a suitable cell to camp on, which will be known to those skilled in the art and will not, therefore, be described in more detail. Unlike normal idle mode search procedures (which can occur, essentially, at any time), however, the cellular network monitoring modules 46 restrict their ‘idle mode’ (or ‘pseudo idle mode’) search procedures, while D2D communication is occurring, to coincide with the above mentioned transmission gaps.