Key telephone system

A key system or key telephone system is a multiline telephone system typically used in small office environments.
Key systems are noted for their expandability and having individual line selection buttons for each connected phone line, however some features of a private branch exchange such as dialable intercoms may also commonly be present.
Key systems can be built using three principal architectures:
Electromechanical shared-control
Electronic shared-control
Independent keysets
Electromechanical shared-control key systems
Before the advent of large-scale integrated circuits, key systems were typically built out of the same electromechanical components (relays) as larger telephone switching systems. The system marketed in North America as the 1A2 Key System was entirely typical and sold for many decades.
Electronic shared-control systems
With the advent of LSI ICs, the same architecture could be implemented much less expensively than was possible using relays. In addition, it was possible to eliminate the many-wire cabling and replace it with much simpler cable similar to (or even identical to) that used by non-key systems. One of the most recognized such systems is the AT&T Merlin.
Additionally, these more-modern systems allowed vastly more features including:
Answering machine functions
Remote supervision of the entire system
Automatic call accounting
Speed dialing
Caller ID
Etc.
Features could be added or modified simply using software, allowing easy customization of these systems.
Independent keysetsLSI also allowed smaller systems to distribute the control (and features) into individual telephone sets that don't require any single shared control unit. Generally, these systems are used with a relatively few telephone sets and it is often more difficult to keep the feature set (such as speed-dialing numbers) in synchrony between the various sets.
PBX systems
The line between the largest key systems and full PBX systems is blurred. In the 1A2 days, the line was clear: 1A2 systems did not allow the sharing of anonymous "trunk" lines and PBX systems did. Modern key systems blur this distinction by often allowing this feature.
Hybrid keyphone systems
Into the 21st century, the distinction between key systems and PBX has become increasingly confusing. Early electronic key systems used dedicated handsets which displayed and allowed access to all connected PSTN lines and stations. The modern key system now supports ISDN, analogue handsets (in addition to its own dedicated handsets - usually digital) as well as a raft of features more traditionally found on larger PBX systems. The fact that they support both analogue and digital signalling types gives rise to the "Hybrid" designation.
The modern key system is usually fully digital (although analogue variants persist) and some systems embrace VOIP. Indeed, key systems now can be considered to have left their humble roots and become small PBXes. Effectively, the aspects that separate a PBX from a key system are the amount, scope and complexity of the features and facilities offered.