The apparatus transmits in a first zone, for example, an aircraft, a masking signal which masks transmissions from a second zone outside of the first. By doing this a mobile cellular telephone is inhibited from connecting or attempting to connect to base-stations on the ground. A hole in the masking signal spectrum may be provided to enable connection to a base-station within the aircraft. Alternatively, a base-station within the aircraft may be arranged to transmit at a power level greater than the masking signal.