A mobile unit (1) is disclosed for use with a cellular radio transmission system in the form of a plurality of base stations (BS1 to 7) adapted for two-way radio communication and situated at respective geographical locations to define a corresponding plurality of overlapping service areas (SA1 to 7) constituting one or more regions. The mobile unit comprises a receiver (5), control means (2) for controlling the mobile unit and a user interface (6, 9, 10) wherein the control means is adapted whereby, in use and having pre-programmed the control means with information identifying a predetermined service area, the control means recognizes entry of the mobile unit into that service area from an adjacent service area and notifies a user of the mobile unit of said entry via the user interface. The mobile unit may further comprising a transmitter (5) and be adapted to communicate by two-way radio with the base stations. The user may be notified with an audible, visible or mechanical alarm by the user interface.