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Direct distance dialing (DDD) or direct dial is a telecommunications term for a network-provided service feature in which a call originator may, without operator assistance, call any other user outside the local calling area. DDD requires more digits in the number dialed than are required for calling within the local area or 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 is a North American Numbering Plan term considered obsolete since completing a call in any manner other than direct dialing became rare. 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. ==History== The telephone industry made a United States "first" in the New Jersey communities of Englewood and Teaneck with the introduction of what is known now as ''direct distance dialing''. 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 exchange name and five digits). The 11 cities and their area codes at that time were: * Boston, Massachusetts (617) * Chicago, Illinois (312) * Cleveland, Ohio (216) * Detroit, Michigan (313) * Milwaukee, Wisconsin (414) * Oakland, California (415) * Philadelphia, Pennsylvania (215) * Pittsburgh, Pennsylvania (412) * Providence, Rhode Island (401) * Sacramento, California (916) * San Francisco (318) – San Francisco required the special area code 318 for temporary routing requirements〔San Francisco and Oakland each had their own separate toll-switches, so calls had to be routed accordingly depending on the final destination. As the telephone equipment used at the time could only handle three-digit translation, the temporary use of area code 318 was required in order to distinguish between the two areas. Area code 318 was temporarily used to specify San Francisco and areas north of the Golden Gate, while calls with destinations in Oakland and the East Bay continued to use area code 415. When the electromechanical card-translator box became available sometime during 1952–53, 6-digit translation became possible and the use of area code 318 was no longer required. Area code 318 was reclaimed for future use and the entire San Francisco Bay Area returned to using area code 415.()〕 Many other cities could not yet be included as they did not yet have the necessary switching equipment to handle incoming calls automatically on their long-distance calling circuits. Other cities still had either a mixture of local number lengths or were all still six-digit numbers; Montreal and Toronto, 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Direct distance dialing」の詳細全文を読む スポンサード リンク
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