Nation states organise telephone systems with 'numbering plans', identifying geographical areas or service operators with number prefixes and corresponding number ranges. Some plans hold — forever-reserved — ranges of numbers varying from one hundred (some states of the USA) though to one hundred thousand consecutively ordered (Ireland). Explicitly for use in film and television programmes, producers pick from the designated ranges. The chosen digits appear fleetingly in films, or frequently over years of a serial.
Enquires were made to the telecommunication regulators of each nation state. All possible numbers for each country with such reserved ranges are ordered and listed in the International Directory of Fictitious Telephone Numbers.
The printed directory is accompanied by an apparently normal looking telephone. This telephone has been reprogrammed and reconstructed to automatically dial the numbers from the Directory, at random. The dialling tone is followed by the sound of numbers being dialled, then the response: a dead tone indicating no number, or a recorded message stating that the number is not in use, from a telephone exchange in any part of the world. The telephone then hangs-up before selecting another number to dial at random.