Direct distance dialing

Direct distance dialling (DDD) is a US term. In the UK, the equivalent term is "STD", or Subscriber Trunk Dialling.

In telecommunication, a 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.

Note 1: DDD extends beyond the boundaries of national public telephone networks.

Note 2: DDD requires more digits in the number dialed than are required for calling within the local area.

Source: from Federal Standard 1037C and from MIL-STD-188