telephone exchange

noun4

1. •
   Any equipment that establishes connections between telephones.
2. •
   The rooms or building housing such equipment (a central office, as it was termed by the Bell System).
3. •
   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".

   • A: You can telephone him at 2719. B: What's the [telephone] exchange? A: LA6. I'm sorry, that's LA6-2719.
4. •
   (historical) 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.

   • This valley didn't have a telephone exchange until 1903, when the leading citizens of this town invested in a company to establish one.