Centrex is an acronym for «Centr»al Office «Ex»change Service, a business telephone service provided by local telcos.

What is Centrex (in computer networking)?

An acronym for Central Office Exchange Service, a business telephone service provided by local telcos. By using a Centrex instead of a Private Branch Exchange (PBX), a business can eliminate the necessity of having its own dedicated switching facilities at its customer premises.

Centrex also eliminates the need for customers to upgrade to expensive new telephones, since existing telephone lines and touch tone phones can be used with it.

This frees the customer from the need to invest in the cost and management of customer premises equipment (CPE).

How Centrex Works

Centrex services partition the switching capabilities of the telco’s central office (CO) equipment and allow a portion of these capabilities to be dedicated to a particular customer. In essence, the business customer is leasing dedicated switching facilities at the CO to enable a large number of employee telephones to be routed through a few telephone lines. All routing of calls to individual employee telephones takes place using the Centrex. Configuration changes can be performed at the CO instead of requiring technicians to visit the customer premises. This can save the cost of installing a local PBX at the customer premises.

Centrex can handle advanced communication features such as internal call handling, inbound and outbound call handling, and multiparty calling. Each individual connected can have customized calling features just as they can with a PBX. Maintenance is entirely the responsibility of the telco central office, which provides around-the-clock support.


Some carriers such as Pacific Bell also offer Integrated Services Digital Network (ISDN) as a Centrex service in addition to standard business ISDN lines.