The next time you’re watching an episode of Sherlock or EastEnders, you might notice phone numbers appearing in the background of certain scenes.
They may be listed on a poster on a pub wall, displayed on a taxi’s door vinyls, or listed in someone’s phone book as the camera pans into a close-up of SIM contacts.
You may be tempted to pause the footage and try dialling the number in question.
However, you’ll almost certainly be greeted by an automated message saying the number you’ve dialled has not been recognised, or hear a shrill series of beeps followed by disconnection.
That’s because the numbers which appear on-screen (and on the radio) are entirely fictitious.
Imitation is the sincerest form of battery
To prevent unwitting individuals having their battery life drained by endless prank calls, the numbers displayed in film, TV and radio productions are imitations.
Ofcom has reserved blocks of numbers in most major cities – especially those with film and TV production facilities – which will never be allocated to businesses or people.
In most British cities, a thousand fictitious phone numbers have been set aside in a block that starts with the geographic area code and then continues 4960.
In Glasgow, any 0141 496 0xxx numbers will be unobtainable if you ring them. In Reading, 0118 496 0xxx will achieve the same result, and so on, from Bristol to Tyneside.
The vagaries of area codes mean the fake numbers allocated to Northern Ireland, Cardiff and London are slightly different, but each still has a thousand numbers available for use.
For programmes and movies not set in a specific location, the 01632 area code has been reserved, giving a million different combinations of fictitious phone numbers.
(Confusingly, this particular code is very similar to the 01642 area code used in Middlesbrough and across Teesside.)
Ofcom rightly recognise that not every number portrayed on-screen can be a landline.
They’ve also retained a bank of mobile phone numbers which will never be allocated, starting in 07700 900.
The same is true for freephone numbers – 0808 157 0xxx – and premium-rate numbers, which can be shown as 0909 879 0xxx.
Nor is this a uniquely British phenomenon.
Eagle-eyed viewers of American programmes may have noticed how often the 555 area code appears on-screen.
As America’s version of our 4960 code, any American phone number can be displayed on-screen as 555 01xx, from emergency service vehicles to concert flyers and Missing posters.
In France, a total of 600,000 numbers have been kept aside for use in audio-visual productions, in six blocks of 100,000 numbers (fake mobiles start 6 39 98, for instance).
So the next time you see a phone number appearing on-screen, resist the temptation to call it. These fictitious phone numbers won’t actually put you in touch with anyone.