The number of places with five digit subscriber numbers and an 01xxx area code has declined rapidly in recent decades.
Possible number formats for UK telephone numbers are as follows: (01xx xx) xxxxx (01xxx) xxxxxx (01x1) xxx xxxx (011x) xxx xxxx (02x) xxxx xxxx 03xx xxx xxxx 055 xxxx xxxx 056 xxxx xxxx 070 xxxx xxxx 07624 xxxxxx 076 xxxx xxxx 07xxx xxxxxx 0800 xxx xxxx 08xx xxx xxxx 09xx xxx xxxx Number ranges starting 01 can have NSN length as 10 or 9 digits.
The 0800 range can have NSN length as 10, 9 or 7 digits.
These six areas have a three-digit area code matching the pattern 11x, with a seven-digit subscriber number, and this is known as 3 7 format.
The first three digits of the local number identifies a small area within the town or city.
Just short of 581 areas use this format, and the area codes range from 01200 to 01998.
A small number of these areas also have a few subscriber numbers that have only five digits (see next section).
This leads to a restriction as to which initial digits can be used for subscriber numbers within those four-digit area codes, e.g.
in the 01387 four-digit area code, subscriber numbers cannot begin with a 3 because 013873 is a separate five-digit area code; likewise in the 01946 four-digit area code, subscriber numbers cannot begin with a 7 because 019467 is a separate five-digit area code.
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.