General Toll Switching Plan

The General Toll Switching Plan was a systematic nationwide effort by the American Telephone and Telegraph Company (AT&T) of organizing the telephone toll circuits and cable routes of the nation, and of streamlining the operating principles and technical infrastructure for connecting long-distance telephone calls in North America. This involved the design of a hierarchical system of toll-switching centers, a process that had found substantial maturity by 1929.

Source: Wikipedia — General Toll Switching Plan (CC BY-SA 4.0)