Key systems are typically used for offices of less than 60 stations. For example, a central unit typically allows users to make calls to another in-office extension, and prevents other users from accidentally picking up a line that is being used.
Key systems require professional installation and maintenance.
All outside telephone lines must connect to the KSU, as well as all inside extensions. Unfortunately, configuring and wiring these phone systems can be nearly as costly as the phones themselves. For a company of more than 60 employees, there are PBX systems. The configuration of a PBX system is totally programmable, so PBX systems can support the most complex features. More recently, the distinctions between the key and PBX systems have become relatively blurred.