Key Service Unit

Why Key Telephone Systems Had Relevancy For So Long

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The Key Telephone System is becoming as obsolete as landline telephones and is becoming ill relevant like the rotary dial telephones. It’s being replaced not just with “the cloud” and Voice over IP; it’s the overall methodology how VOIP works today. The other factor is technical “professionals” who often trash customers for retaining older ideas and if anyone insists what they want they’ll be gaslighted as “stuck in the past”.

But this type of telephone system was the earliest type of system that had buttons, had the ability to buzz, ring, lock out another line if the other telephone was busy (privacy modes), and other dodads like Music On Hold and paging.

It’s easy from a historical point of view, that plain ol dial sets in offices were tied to PBX systems and multi button telephones were tied to Key Systems. Of course this is prior to the 1970s before these became electronic devices. Earlier key systems were known as “1A” series, followed by “1A1” and “1A2” systems as the years went on, these were originally developed by Western Electric, but was cloned by other vendors.

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Telephony 101: Key Telephone System (KTS)

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The first on site customer systems was the PBX, but later on in the 1920s into the 1930s, brought a specific solutions to customers, called the Key Telephone System.

The “key” to this was to have hard wired lines for users who wanted to get to someone directly. This also fused with Intercom systems, acting as a separate “service” for the time. Intercoms for this context was a private circuit to enable telephone calls to work within the system. Most often, the Intercom line was on the fifth button of say a 564 telephone.

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Telephony 101: Key Service Units

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In the 1960s, customers of Key Telephone Systems or KTS units, were able to add onto their phone system, such as adding private lines, private trunks, modern intercom and other features alike.

An Automatic Electric KSU with the cover open
(Found from a PDF catalog I found online.)

 

Key Service Units acted like apps/appliances/snapins/plugins of yesteryear. Most often KSUs lived in a shoe box sized unit, a 66 block, a couple fuses, a 3 pronged power cable pigtailed directly to some hardware, and a unit to place features in a circuit breaker sized device. These would be tied to the 66 block where then wiring could be terminated for other goodies like overhead paging, music on hold, link the Muzak service in, etc.

This is also where the trunks from the central office would feed into, and the telephones that would connect.

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