Hertfordshire; and Ewell, Molesey, Thames Ditton and Whyteleafe in Surrey. There are eighteen exchanges within Greater London that do not use the 020 code. The six boroughs that have significant areas within other codes are Bexley, partly within the Dartford (01322) code; Bromley, partly within the Orpington (01689) and Westerham (01959) codes; Croydon, partly within the Orpington (01689) and Redhill (01737) codes; Enfield, partly within the Waltham Cross (01992) code; Havering, mostly within the Romford (01708) code; and Hillingdon, partly within the Uxbridge (01895), Slough (01753) and Watford (01923) codes. Number allocation With the introduction of the (020) area code, as part of the Big Number Change, subscriber numbers were changed from 7-digits (xxx xxxx) to 8-digits (xxxx xxxx). This allowed new ranges of numbers to be issued. Under the National Telephone Numbering Plan the code operates with the following sub-ranges: In 2006 59% of numbers within the 020 code were allocated to BT. The area code is not subject to number conservation and the regulator Ofcom does not restrict the size of number blocks that are allocated. Notes References External links List of exchanges in 1966 Number allocations 20 Infrastructure in London