 Subscriber trunk dialling

Subscriber Trunk Dialing (STD) is a telecommunications term used primarily in the UK and other countries to refer to the system that allows telephone subscribers to make long-distance calls without the need for an operator to assist them. This capability was first introduced in the 1950s and greatly simplified the process of making long-distance calls. In an STD system, callers can dial a prefix (often a trunk code) followed by the area code and the local number they wish to reach.