Centrex is a business telephone service offered by a local telephone company from a local central office. Centrex is basically single line telephone service delivered to individual desks (the same as you get at your house) with features, i.e. "bells and whistles," added. Those "bells and whistles" include intercom, call forwarding, call transfer, toll restrict, least cost routing and call hold (on single line phones).
Centrex is known by many names among operating phone companies, including Centron and Cenpac. Centrex comes in two variations - CO and CU. CO means the Centrex service is provided by the Central Office. CU means the central office is on the Customer's premises.