Direct distance dialing

Direct distance dialing (DDD) is telecommunication service feature in which a caller may, without operator assistance, call any other user outside the local calling area. Direct dialing by subscribers typically requires extra digits to be dialed as prefixes than for dialing within the local area or within an area code. DDD also extends beyond the boundaries of national public telephone network, in which case it is called international direct dialing or international direct distance dialing (IDDD).

DDD was the term used when the North American Numbering Plan was implemented in the 1950s. In the United Kingdom and other parts of the Commonwealth of Nations, the equivalent terms are or were "STD", for subscriber trunk dialing, and "ISD" for international subscriber trunk dialing.

The first direct-dialed long-distance telephone calls were possible in the New Jersey communities of Englewood and Teaneck starting on November 10, 1951. Customers of the ENglewood 3, ENglewood 4 and TEaneck 7 exchanges, who could already dial New York City and area, were able to dial 11 cities across the United States, simply by dialing the three-digit area code and the seven-digit number, which at the time consisted of the first two letters of the central office name and five digits.

The eleven destinations at that time were:

Many other cities could not yet be included as they did not yet have the necessary toll switching equipment to handle incoming calls automatically on their circuits. Other cities still had either a mixture of local number lengths or were all still six-digit numbers; Montreal, Quebec and Toronto, Ontario in Canada, for example, had a mix of six- and seven-digit numbers from 1951 to 1957, and did not have DDD until 1958. Whitehorse, Yukon, had seven-digit numbers from 1965, but the necessary switching equipment was not in place locally until 1972.

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