The Big Number Change addressed various issues with the telephone dialling plan in the United Kingdom, during the late 1990s and early 2000s, when the country was running short of new telephone numbers.
The change greatly expanded the pool of available numbers within those places while retaining 'local dialling' (the ability to dial local numbers directly, without needing to dial an area code first).
The change affected the dialling codes assigned to Cardiff, Coventry, London, Northern Ireland, Portsmouth and Southampton.
The other set of changes affected mobile, non-geographic and premium rate numbers, completing a series of steps first detailed almost a decade earlier.
Existing mobile, non-geographic and premium rate services continued as before, using various codes from 02 to 09 and with nine-digit numbers.
Within each of these groups some smaller number blocks were held aside for the older 9-digit mobile, non-geographic and premium rate services to move into at a later date.
The Big Number Change put those final parts into place.
Normal practice in Great Britain was to allocate one area code per charging group but, as calls to Northern Ireland were national rate from anywhere in GB, it was not seen to be necessary to differentiate between all of them.
As a result, at the Big Number Change, all these dependent exchanges were migrated to different numbers than their parent exchanges; in this instance while Enniskillen's 01365 (the 1 coming from PhONEday) was migrated to 028 66, Kesh went from 013656 to 028 686 and Lisnaskea from 013657 to 028 677.
The 0500 range was eventually ceased on 3 June 2017, with numbers transferred to 0808 5xx xxxx equivalents during a three year transition period prior to that date.
These fictional numbers were originally made available by Oftel for dramatic purposes; for example, for quoting in TV, radio and film programmes.
There is a widespread but erroneous assumption that London has several area codes – 0203, 0204, 0207 and 0208 – whereas, in fact, it has just one: 020.