The 0845 range can have NSN length as 10 or 7 digits. There are no telephone numbers in the UK with an NSN length of 8 digits. It has a four-digit area code (after the initial zero) and a six digit subscriber number, and is known as 4 6 format.These area codes were changed by adding a "1" directly after the initial zero as a part of Ph ONEday in 1995.The Big Number Change removed many, especially in Northern Ireland, and by July 2005 there were only 329 ranges in 42 codes.By April 2010 this had reduced to 324 ranges in 40 codes, with the same number still in November 2012. This is the geographic number format for the first round of five large cities moved to all figure dialling in the 1960s, and subsequently also used in the metropolitan county of Tyne and Wear, eastern County Durham and south-eastern Northumberland from the 1980s onwards.The former Reading area code had already been changed once, by adding a "1" directly after the initial zero as a part of Ph ONEday in 1995. It is used for the third tier of large cities and for Northern Ireland, and was formed as a part of the Big Number Change in 2000.
The numbers therefore have only nine digits after the initial zero trunk code.
The "area code" is also referred to as an "STD (code)" (subscriber trunk dialling) or a "dialling code" in the UK.
The code allocated to the largest population is (020) for London.
The overall structure of the UK's National Numbering Plan is: In the United Kingdom, area codes are two, three, four, or, rarely, five digits long (after the initial zero).
Regions with shorter area codes, typically large cities, permit the allocation of more telephone numbers as the local number portion has more digits.