Centrex

It provides functions similar to a PBX, but is provisioned with equipment owned by, and located at, the telephone company premises.

[2] This accounted for approximately 5% of all installed business telephone lines, worldwide.

[2] In terms of user-visible features, Centrex and PBX are similar.

Features include:[4] In the United States, the usage of Centrex lines has fallen from 16.5 million in 2002 to 10.7 million in 2008 as users transition to IP-PBX (through VoIP).

[5][6] Centrex continues to be used by large institutions, government agencies, and universities as most of the equipment has already been paid for, though leasing Centrex lines may be more expensive.

Typical standard phone used with Centrex. Note the "Recall" button and the Message Waiting Lamp.