Direct distance dialing
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In telecommunication, direct distance dialing (DDD) is 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. DDD also extends beyond the boundaries of national public telephone networks.
Direct distance dialing (DDD) is a North American Numbering Plan term, and considered obsolete as calls are now rarely completed in any other manner. In the UK and other parts of the British Commonwealth, the equivalent term is or was "STD", for subscriber trunk dialling.
Source: partly from Federal Standard 1037C and from MIL-STD-188