Subscriber trunk dialling (STD) is an obsolete term for the UK telephone system allowing subscribers to dial trunk calls without operator assistance. 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, or was, also used in other countries such as 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 that the STD system was completed ([2]). The system required that each area have its own STD code which could be dialled by subscribers, and although they are now officially 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 ([3]).

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.

See also

UK telephone numbering plan

References

http://www.btplc.com/Corporateinformation/BTArchives/history/Eventsintelecommunicationshistory.htm

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.

http://news.bbc.co.uk/onthisday/hi/dates/stories/may/21/newsid_2510000/2510289.stm

Archive news article from the BBC on the introduction of Subscriber Trunk Dialling.

http://www.sigtel.com/tel_hist_earlystd.html

Article detailing how the original STD Codes were allocated.