Wikipedia
PhONEday was a change to the telephone dialing plan in the United Kingdom on 16 April 1995. It changed geographic area codes and some telephone numbers. In most areas a "1" was added to the dialling code. In Bristol, Leeds, Leicester, Nottingham and Sheffield the area codes were replaced with new codes and the subscriber numbers gained an extra digit. The PhONEday changes also made provision for new ranges of subscriber numbers in those five cities. A £16m advertising campaign, and an eight-month period of parallel running during which old and new codes were active, preceded the change. PhONEday followed a change made in May 1990 when the old London area code 01 had been released from use, permitting all United Kingdom geographic numbers to begin with this prefix. Originally planned in 1991 to take place in 1994, in 1992 the change was postponed until 1995.
The PhONEday changes also released space for new geographic area codes beginning 02, which would come into use as part of the Big Number Change in 2000. The changes also allowed 10-digit numbers beginning 07, 08, and 09 to be used for mobile, non-geographic, and premium-rate services, from 1997 onwards, with all remaining 9-digit mobile, non-geographic, and premium-rate numbers from 02 to 09 being converted to 10 digits and moved into the 07, 08, and 09 prefixes in 2001.