Its intercommunication ability allows two or more stations to directly connect while not using the public switched telephone network.Please help improve this article by adding citations to reliable sources.
![]() With a key telephone system, a station user could control the connections directly using line buttons, which indicated the status of lines with built-in lamps. The earliest systems were known as wiring plans and simply consisted of telephone sets, keys, lamps, and wiring. The 1A family of Western Electric Company (WECo) key telephone units (KTUs) were introduced in the late 1930s and remained in use to the 1950s. A equipment was primitive and required at least two KTUs per line; one for line termination and one for station (telephone instrument) termination. The telephone instrument commonly used by 1A systems was the WECo 300-series telephone. ![]() As the 1A1 systems became commonplace, requirements for intercom features grew. The original intercom KTUs, WECo Model 207, were wired for a single talk link, that is, a single conversation on the intercom at a time. The WECo 6A dial intercom system provided two talk links and was often installed as the dial intercom in a 1A1 or 1A2 key system. The 6A systems were complex, troublesome and expensive, and never became popular. The advent of 1A2 technology in the 1964 simplified key system set up and maintenance. These continued to be used throughout the 1980s, when the arrival of electronic key systems with their easier installation and greater features signaled the end of electromechanical key systems. 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. Electronic shared-control systems led quickly to the modern hybrid telephone system, as the features of PBX and key system quickly merged. 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. Early electronic key systems used dedicated handsets which displayed and allowed access to all connected PSTN lines and stations. Effectively, the aspects that distinguish a PBX from a hybrid key system are the amount, scope and complexity of the features and facilities offered. These advanced hybrid key systems generally only require attached PBXs for interaction with back-office staff and voicemail. These systems commonly have their front end units referred to as Turrets and are notable for their presentation of hoot-n-holler circuits. Multiple Hoots are presented to multiple users over multiplexed speakers to multiple locations. ![]() September 2017 ) ( Learn how and when to remove this template message ).
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