Abstract: A mobile radio tarnsmission system comprises a plurality of base stations (BS) with corresponding service areas (SA) which together cover a geographical area through which mobile stations (M) can roam. Each base station is connected to a central station (CS) by means of a respective two-way communication link (CL). In order that it can be contacted without all the base stations having to put out a call, each mobile station registers in known manner with the base station in whose service area it is situated, and this is recorded by the control station. When it roams into a new service area this process is repeated. However, in order to preclude frequent re-registrations when the mobile station repeatedly crosses the boundary between two service areas, when it registers with a new base station and hence a new service area it retains its registration with at least one service area with which it is already registered, thereby creating its own customized enlarged registration area.