Subscriber trunk dialling

Subscriber trunk dialling (STD), also known as subscriber toll dialing, is a telephone numbering plan feature and telecommunications technology in the United Kingdom and various Commonwealth countries for the dialling of trunk calls by telephone subscribers without the assistance of switchboard operators. Switching systems to enable automatic dialling of long distance calls by subscribers were introduced in the United Kingdom on 5 December 1958, using destination codes that were based on the letters in a town's name.

Source: Wikipedia — Subscriber trunk dialling (CC BY-SA 4.0)