Key Systems

A key system is a kind of multi-line telephone system often used in small offices. In a key system, all lines are directly connected to the telephone company, unlike a PBX system, which has a main unit at the customer site. Because a key system connects to main telephone company lines, users do not have to dial a special number to make outgoing phone calls.


Key Systems' Capabilities

Key systems can easily be expanded, and are ideal for office use because they allow any employee to access any line. Employees can also transfer calls, have conference calls, make interoffice and outside calls, and use the phone system as an intercom. Key systems can be integrated with other features and equipment, such as Caller ID, voice mail, and "on-hold" music.

Types of Key Systems

There are four primary types of key systems:

  • Electromechanical shared-control key systems
  • Electronic shared-control key systems
  • Independent key sets
  • Hybrid phone systems.

Electromechanical Shared-Control Key Systems

The earliest type of key system, the electromechanical shared-control system, used the same kind of electromechanical relays as larger telephone systems. These early key systems consisted of a main control unit linked to several specialized telephone sets. Each telephone line was controlled by three pairs of wires: the first pair carried the phone line, the second pair carried the information for that line, and the third pair linked the line to a lighted button on the telephone. With these lights, users could determine which lines were idle, which were in use, which were ringing, and which were on hold. To access a specific phone line, the user simply had to press that line's corresponding button. Electromechanical shared-control systems supported simple additional features, such as intercom lines, manual buzzers, and "on-hold" music.

Modernized Key Systems

As new technology was developed, these systems were replaced by simpler, less expensive systems called electronic shared-control key systems. These systems supported more complex functions, such as answering machines, speed dial, caller ID, and remote control of the entire system. They were also easily modified, by simply using software to add or alter features. The same new technology that contributed to the electronic shared-control systems also made it possible to set up a system without a main control unit. These systems used independent key sets, and because there is no central control unit, it can be difficult to keep the individual telephones synchronized. Today, many key systems have surpassed their simple beginnings, and are hybrids of traditional key systems and new technology like VOIP.