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Subscriber trunk dialling (STD, also known as subscriber toll dialling) is a term for the UK telephone system allowing subscribers to dial trunk calls without operator assistance. The introduction in the UK of subscriber dialling of long distance calls removed the distinction that had existed between Trunk and Toll calls. This term however, is still widely prevalent in India to describe any national call made other than one's local unit. A "subscriber" is someone who subscribes to, i.e. rents, a telephone line and a "trunk call" is one made over a trunk line, i.e. a telephone line connecting two exchanges a long distance apart. Now that all calls may be dialled direct, the term has fallen into disuse.

The term "subscriber trunk dialling" is used in other countries such as the Republic of Ireland and Australia, but the corresponding term in the US and Canada is direct distance dialing.

In the UK, STD started when, on 5 December 1958, the Queen, who was in Bristol, dialled a phone call to Edinburgh ([1]). However, it was not until 1979 ([2]) that the STD system was completed. The system required that each area have its own STD code which could be dialled by subscribers, and although they are now sometimes called area codes, it is still common to see and hear the old term in everyday use.

The term was extended, when on 8 March 1963, London subscribers were able to dial Paris direct using international subscriber trunk dialling.

In India the term still applies and to dial out of one's unit area, the relevant city code has to be prefixed to the local number. This prefix starts with a 0. Certain numbers are also prefixed with 95 which are local calls within 500 km.

• UK telephone numbering plan
• list of country calling codes

• The archives of BT including archives of its predecessor organizations: A good source of information relating to the history of the telephone system in the UK.
• Archive news article from the BBC on the introduction of Subscriber Trunk Dialling.