A local telephone company switching center. There are two types of central offices (COs). The first is called an "end office" (EO) or "local exchange" (LE) and connects directly to the outside plant, which is the feeder and distribution system to homes and offices. The end office (often called a "Class 5 office") provides customer services such as call waiting and call forwarding.
The second type is the tandem office (also toll office or tandem/toll office), which is a central office that does not connect directly to the customer. Toll call record generation and accounting used to be handled in the tandem offices. Today, the billing is mostly done in the end offices. There are more than 25,000 central offices in the U.S. See LEC
End offices provide the local loops directly to customers, while tandem offices carry traffic between end offices. The PSTN is made up of local exchange carriers (LECs) and interexchange carriers (IXCs) that are governed by LATA boundaries.
High-Tech Switching in the 19th Century
This "central office" was used to switch phone calls between all the lawyers in Richmond, Virginia in 1882. (Image courtesy of AT&T.)