This paper describes a prototype system, ZoneIT, for selectively controlling functionality of mobile devices, e.g. ringing of cellular telephones. ZoneIT exploits the fact that many such devices have short-range radio communication systems (e.g. Bluetooth) to allow these devices to communicate with Base Stations to learn what functionality is permissible in a zone. ZoneIT is secured by a process that enables mobile devices to authenticate base stations, and so protect against denial of service attacks.