to work in parallel with new codes until January 1970, when the "ANN: All-figure Numbers Now" advertising campaign prompted callers to use only the new codes. The transition to all-figure dialling allowed the codes to be grouped into eight sectors; all exchanges within 6 km of the centre of London formed the Central sector and the other sectors radiated from it.

The first three digits of a subscriber number continued to indicate the exchange to which the number belonged; for example, "222 1234" was in Westminster (Central Sector) and