Method of operating a dual mode mobile telephone

A method of operating a mobile telephone within a geographical area having overlapping coverage from a first, GSM, network and a second, UMTS, network, where said GSM network is the home network (HPLMN) of the mobile communication device and said GSM and UMTS networks provide respective first and second sets of services in said geographical area. The method comprises registering the telephone with the GSM network when a service of said first set is requested and registering the device with the UMTS network when a service of said second set, but not of said first set, is requested. The method provides the mobile telephone with a roaming for service function.