CENTRAL OFFICE

A Central Office is the location within a city where the main telephone lines go and where there exists electronics to switch telephone calls from one line to another.  Not surprising, the giant electronics that does switch telephone calls is also called a "Switch".  There will usually be one or more electronic switches in the central office.

When cellular calls are to be sent over normal phone lines, as most cellular conversations are for at least part of their journey, the cellular base stations must connect to the central office.  The means of connecting the base stations at cell sites to the central office is called "backhaul" and is often done over wire lines such as T1, T3, or E1 wire lines.