Central Exchange (or Centrex) is a service provided by many telephone carriers to deliver PBX or Key-like services via the telephone company (or Telco.) Little to no hardware is required at the customer end, unless its special adjuncts for digital telephones for power, etc. Regardless of the hardware, the “brain” lives out in the “cloud” so to speak at the central office. If it’s a government, or a large corporation, rarely are these actually installed on site.