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.
(No pun here) but here’s some key points:
- Key systems had basically a finite number of phone lines and extensions. It was more phone centric, than lines, these shoeboxes had 6 lines, and up to 16 “stations”, some could scale from 8 lines to 24, and some could go up to 100 stations and 40 lines.
- For virtually all Key Service Unit installs, depressing on the specific line button would open up that line automatically, and you’d either dial the number or hit “9” if it was against a PBX or Centrex setup.
- To compare to 21st Century data technology, the best metaphor was the KSU was the “gateway” and the PBX was more of a core router.
- If you had seen a multi button phone in an office setting, say a large campus, it most likely was tied into a KSU and piggybacking off a PBX on an analog extension number, and each line into the KSU came from a station port on the PBX.
- You rarely saw plain ol 500 rotary or 2500 phones in a KSU setup. It was designed to have Call Director type phones, or the 10 button, 20 button or 30 button, push button line sets.
Broadcast operations had used KSUs up until recently in the mid 2010s, some were still using 1A2 type of KSUs even into the new millennia. Part of this is, especially for news gathering, if an important source or story needs to be patched from the newsroom to the control room, transferring a call just wouldn’t work, wrong numbers are likely to happen, etc. For one KSU telephone line, it could be tied to other KSUs, more than several.
Key systems are ideal for direct access to customers in retail. For smaller retail, key systems have been critical. Only “anchor” department stores, or large box stores would have a PBX.
With the advent of the Integrated Circuit, by the 1970s, the ability to write complex code to emulate the moving electromechanical beast, into a smaller unit, would take place and by the 1980s, Nortel (nee. Northern Telecom), AT&T, and the Japanese would woo for various customers as electronic key telephone systems or EKTS were becoming a popular product. For the first time, many places that had a PBX, could migrate to an automated PBX, that had phones seen in smaller systems. PBX phones prior to the 1980s were basically the ol 2500 and 500 sets. Some of the said companies had strikes and misses, but overall by the 1990s, you didn’t need to say “E” in EKTS, because they were all electronic.
The smaller end “phones” had a lot of features that helped users make use of they EKTS systems even more, especially when the lines of PBX and KTS systems were blurred. Sets used on Nortel’s Meridian PBX for an example is modeled more like a 1A2 phone, than say a multi appearance phone you see in say the AT&T>Avaya world; and in that latter part of the Avaya world, the PBX stations trailed behind the functionality of a Merlin set.
In many early implementations of Voice over IP, the legacy vendors did migrate their Key systems into the world of VOIP, but now with Session Initiation Protocol becoming the standard, either SIP or the PBX-metaphor has been pushed as the only way in a “cloud” world. SIP can function as a landline, or a PBX set. The reasoning is the “stack” and it’s limitations due to how the people who invented it had a different set of minds. If your SIP phone has a “key” or “shared line appearance” all it does is give you indirect access, but not the direct access you would get on a legacy system, so you can’t tell if that dialtone is coming from your carrier and not your SIP phone.