Abstract: A mobile subscriber unit is provided with a memory (8) containing the cell selection and re-selection functions relating to a plurality of different radio networks (4 to 6) eg. GSM, UMTS, satelite, available in its locality, thus enabling the subscriber unit to identify and camp on to any one of the networks. Once a communications link is established, the subscriber unit requests over-the-air download of the full operating protocol from the chosen network. A self-check procedure is optionally provided to ensure that the downloaded data is fuctioning correctly. The choice of network may be influenced by user preference data held in a memory (10). The invention enables operation of a subscriber unit on a choice of many networks without the need for a large memory for holding the full protocols for all available networks.