The 'Big Number Change' addressed various issues with the telephone dialling plan in the United Kingdom, during the late-1990s and early-2000s.

The first was an update to a small number of geographic dialling codes in response to the rapid late-1990s growth of telecommunications and impending exhaustion of local numbers in several cities. 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, culminating in a large switch on 22 April 2000. All of these places moved to eight-digit local numbers ensuring sufficient local capacity for many decades (London saw a five-fold increase in capacity, for example). No other geographic area codes were affected.

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