Subscriber trunk dialling (STD, also known as subscriber toll dialling) is a telephone system allowing subscribers to dial trunk calls without operator assistance. The term was introduced when it first became possible for long-distance calls to be dialled directly, and is now rarely used where calls to any destination can be dialled.
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 term was extended when, on 8 March 1963, subscribers in London were able to directly dial Paris using international direct dialling.
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 outside 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.
When telephone systems were first introduced, subscribers called a telephone exchange and asked a human operator to connect the call to another subscriber on the same exchange; calls to other exchanges were originally not possible. Later it became possible to dial numbers on the same exchange; calls to other exchanges (trunk calls) were possible, but had to be connected by an operator. When subscribers in one area became able to dial non-local subscribers, the term used for the innovation was subscriber trunk dialling.
In the UK, STD started before 5 December 1958 when the Queen, who was in Bristol, publicized it by dialling 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 a new STD code, which could be dialled by subscribers, be allocated to each area; in the UK area codes are still sometimes called STD codes.
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. Codes were later changed (e.g., London became 020, and Manchester 0161).
Until 1992, calls to these cities from the Republic of Ireland required the following codes:
In that year, this changed to dialling in the international format 0044, and the 03 range was withdrawn from use.