History of Area Codes in the United Kingdom
The increased demand for telephone numbers due to the introduction of fax machines, mobile phones and organisations wanting to provide employees with direct telephone numbers has resulted in the need for the area codes having to change a number of times to provide additional capacity.
There have been 2 major changes to Area Codes since their introduction in the 1960s and a number of smaller changes, sometimes only affecting a single Area Code. We will cover these changes below, but lets start by looking back to the time before exchange codes existed.
Operator Dialing
When telephone systems were first introduced in the United Kingdom, a subscriber had to call the local telephone exchange and tell the human operator who they wanted to be connected to. Originally, it was not possible to call someone on another exchange, so calls could only be made between subscribers in a small local area sharing the same exchange.
Eventually, it became possible to for subscribers to call other exchanges, again going through a human operator and to make local calls directly without the need to use the operator. In order for a subscriber to dial someone on another exchange (a so called trunk call) they would need to first dial the dialing code of that exchange and then the subscriber number. Initially, the dialing codes were 3 represented by at least 3 character codes, which were dialed after an initial '0', which gave trunk access. In the majority of towns and cities, the dialing code was usually the first 2 characters of the town name, followed by a digit and the alphabetical characters were represented on the rotary dial of the telephone. So, for example, the code for Bath was 'BA5', which would be dialed as '0225'. In the larger cities, there were multiple exchanges so the dialing code would depend on the area of the city, for example in London, the Wimbledon exchanage was identified as WIM and dialed as 946.
Numeric Codes and "ANN: All-figure Numbers Now"
Starting in 1959 with the allocation of the STD code '01' to London and continuing until 1966, the previous mnemonic codes were altered to be totally numeric. In most areas, this meant no change to the actual code as it would use the numeric values of the previous codes, so dialing Sevenoaks for example, changed from '0SE2' to the numeric equivalent '0732'. Some of the cities continued to allow the old and new codes to work in parallel until 1970 and the "ANN: All-figure Numbers Now" advertising campaign, which prompted callers to only dial the new numeric codes.
For 20 years, this is how numbers remained, London with its '01' code, the cities of Birmingham, Edinburgh, Glasgow, Liverpool and Manchester with 2 digit codes where the second digit depresented the first letter of the cities name ('021' for Birmingham, '031' for Edinburgh, etc) and smaller towns and cities with their 4 digit codes such as '0732' for Sevenoaks and '0225' for Bath.
1990 London Splits in two
Initially, mobile phone numbers had just been allocated to ranges of numbers with unused dialing codes, which looked no different than the geographical numbers. The increased demand for numbers caused by mobiles, faxes and direct lines within organisations resulted in projections of number exhaustion in certain areas.
As a first step to alleviate this problem and come up with a standardisation of the number ranges, in May 2990, London's '01' code was abandoned and the area divided between '071', which covered inner London and '081' covering outer London. Not only did this change potentially double the number of available subscriber numbers, but it also freed up the '01' code for the next phase of the reorganisation.
1995 PhONEday
On 16 April 1995, the largest change to area codes since the introduction of the numeric codes in the 1960s took place. In the majority of areas, a '1' was added to the number after the initial '0'. This meant that the code for Sevenoaks changed from '0732' to '01732'. In the cities of Bristol, Leeds, Leicester, Nottingham and Sheffield the old area codes were replaced with brand new 4 digit codes and the subscriber numbers increased by a digit. So, Bristol for example saw its numbers change from '0272 123456' to '0117 912 3456'.
Following the change, the codes '02' to '09' had been cleared of geographic numbers and were available for use for mobile, non-geographic, and premium rate numbers. Initially, the pre-existing 9 digit mobile , non-geographic and premium rate numbers remained in place and from 1997 new numbers for those services were allocated with 10-digits conforming to the new numbering plan.
2000 The Big Number Change
The Big Number Change completed the consolidation of numbers by prefix that PhONEday had started 5 years earlier. This was achieved by 2 major changes.
The first change was to the area codes for Cardiff, Coventry, London, Northern Ireland, Portsmouth and Southampton, which were moved from their previous codes that started with '01' to a brand new series of 3 digit codes starting with '02'. The result of this was to massively increase the availability of numbers in these areas, which was necessary due to increased demand in the late 1990's.
The second part of the change was to move all of the remaining legacy non-geographic and premium rate numbers that were scattered across various dialing codes so that they all started with either '07', '08' or '09' depending on the type of service.
After The Big Change was introduced on 22 April 2000, all of the UK's geographic numbers were in the ranges '01' and '02', VoIP and Corporate non-geographic numbers in the '05' range, mobile and personal numbers were in the range '07', special service numbers such as freephone and revenue-share numbers started '08' and premium rate numbers all started '09'. This left the ranges '03', '04' and '06' available for future expansion, although '03' has since been allocated to non-geographic landline numbers in 2007.