Subscriber trunk dialling
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The term subscriber trunk dialling is used in the United Kingdom, the Republic of Ireland, Australia, India and South East Asia. The corresponding term in the North American Numbering Plan, e.g., in the United States and Canada, is direct distance dialing.
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.
In the UK, STD started on 5 December 1958 when the Queen, who was in Bristol, dialled a phone call to Edinburgh – the farthest distance a call could be directly dialled. The STD system was completed in 1979, though most of the country was covered well before then. 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.
Numbering plan 
With the introduction of subscriber trunk dialling each city with a Director system was assigned a 3-digit code in which the second digit corresponded to the first letter of the city name on the telephone dial, with the exception of London which had the two-digit code 01:
- 01 London
- 021 Birmingham
- 031 Edinburgh
- 041 Glasgow
- 051 Liverpool
- 061 Manchester
Calls from Ireland 
Until 1992, calls to these cities from Ireland required the following codes:
- 031 London
- 032 Birmingham
- 033 Edinburgh
- 034 Glasgow
- 035 Liverpool
- 036 Manchester
In that year, this changed to dialling in the international format 0044, and the 03 range was withdrawn from use.
See also 
- Trunk prefix
- Telephone numbers in the United Kingdom
- Telephone numbers in India
- List of country calling codes
- "Events in Telecommunications History: 1958, 'BT's history', btplc.com/". 2008-06-18. Retrieved 2008-08-12.
- "Events in Telecommunications History: 1979, 'BT's history', btplc.com/". 2008-06-18. Retrieved 2008-08-12.
- STD codes in India
- 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 Dialing.
- BBC video of first call taking place
- Find All India STD Codes