The invention relates to a method and device for operating a mobile communications system, which comprises a multitude of radio cells and at least one mobile communications terminal with a subscriber identification module, the operation ensuing according to the whereabouts of the communications terminal. According to the invention, at least one subscriber area is established within a geographic total area covered by the mobile communications system and determines those radio cells that are located at least partially within the subscriber area. The cell identifications of the determined radio cells are transmitted to the communications terminal and subscriber identification module of the subscriber and are stored there. A comparison then ensues of the correspondence of the cell identifications of the radio cells, which currently serve the communications terminal, with the cell identifications stored in the subscriber identification module whereby, if appropriate, a special operating mode is switched to that is assigned to the subscriber area.