(proper) A telephone system for businesses, large organizations, etc. in which outside calls can be made directly to, or from, any extension.
A telephone service in which the PBX is located in the telephone company's facilities. Some CENTREX services provide the PBX switching at the customer's site, but control is still in the central office. See IP CENTREX.
A contraction of Central exchange.A service that provides PBX-like features from a central office (CO) via a special software load. Centrex service generally is provided from the CO via a dedicated voice grade local loop connected to each voice telephone set or other terminal located on the customer premises. Although it is unusual, Centrex service also can be provided by locating a remote CO partition or a remote line shelf on the customer premises, and connecting it to the main CO with one or more high capacity circuits. Centrex first appeared in the early 1960s in the United States and Canada, were it was de-emphasized it in the 1970s in favor of PBXs, but regained popularity in the mid-1980s. As a CO-based service, Centrex is primarily offered by the incumbent local exchange carriers (ILECs), also known as telephone companies. The emergence of IP Centrex in the mid-1990s has expanded Centrex offerings to competitive service providers providing IP-based services over the Internet. Proprietary voice terminals (P-phones) are required for ease of feature access, although most systems support generic sets, as well. P-Phones are switch-specific. See also IP Centrex and PBX.