telephone exchange

Short definition from Wiktionary

Noun

1. 1
   Any equipment that establishes connections between telephones.
2. 2
   The rooms or building housing such equipment (a central office, as it was termed by the Bell System).
3. 3
   A local or locoregional network of such switched connections, of which a central office was the hub; the local company formed to create and maintain this network and its central office.
4. 4
   The portion of a telephone number representing that network, among the other networks of the larger system: the central office code. In the North American Numbering Plan, this is the "555" portion of "+1-212-555-6789".