Central office
In telecommunications, a central office, or CO, is a facility where individual telephone lines in a limited geographic area are connected to the public telephone network.
Local telephone lines are connected to a central office over what is called a local loop. This loop handles analog, DSL (digital subscriber line), and ISDN (Integrated Services Digital Network) signals, all of which may be switched between local and long-distance services. Altogether, there are over 25,000 central offices in the United States.
Central office types
COs come in two varieties. The first is the end office (also known as a local exchange). It connects to the public telephone network and provides customer services like call waiting and call forwarding. The second is called a tandem office. Its function used to be handling call records and billing for phone companies.