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.
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.
By the 1970s, this technology would become out of date with the rise of electronics, despite these systems living for at least another decade for some. AT&T had introduced a concept of a “KSU-less” system called the ComKey in the 1970s. The system could handle up to 4 lines and 16 extensions and would require at least one “slave” phone and other “master” phones that would go along.
However the ComKey only handled basic stuff. Wanted more features? MOH, Loudspeaker paging and alike required a shoebox like backend to the broom closet. The ComKey to say the least, made it easier to wire the phone sets and kinda the ancillary services unlike it’s predecessor.
KSU-less systems are very common in small business setups today, with phones you can find from your local Staples, with very limited features, and the ability to do intercom, etc. These are the modern day ComKeys.
But for loudspeaker paging, Music on Hold, barring specific numbers, required more brains, and it couldn’t live on phones. In the 1980s, Nortel’s Vantage, AT&T’s Merlin and Mitel’s SX-50 introduced features that could fit on that same “shoebox” form factor but fill it up with brains and jacks instead. Enclosed, plastic boards with 50-pin female connectors replaced the circuit breaker like plugins and introduced a lot more features other than paging, intercom, MOH, etc.
In the 1990s, the rise of digital telephony enabled newer generations to support ISDN trunking, Caller ID, Computer Telephony Integration, etc. The systems at this point were Nortel’s Norstar, AT&T’s Partner (under 48 phones), Merlin Legend (under 200) and Mitel’s improved SX-50 along with the Japanese competition from NEC and Panasonic. In the 1990s, 80% of the US economy was customers under 50 employees, also factor large corporations with remote sites like retail, this allowed even more customers to acquire systems that was once only for enterprises. The Norstar was actually code that fit on a monstrous sized digital PBX, known as the SL-1 more than a decade before and could fit into a shoebox form factor.
With the rise of more applications, KSUs became more like miniature PBX systems despite the ability to function like a KTS. The ability to scale like a PBX to handle more lines over phones or vice or versa was more of an option. The ability to call your local Dominos and have your address and name appear every time you called was a benefit of CTI or Computer Telephony Integration. This same technology enabled you to contact your pharmacy and refill your prescription. This was fed through light duty digital telephone lines to link the personal computer to the telephone system. (Interactive Voice Response or IVR has been the equivalent to call centers with PBX systems.)
The rise of Voice Over IP in the early 2000s, made the KSUs easier to obsolete, by replacing PC sized hardware to Netgear or Linksys-like sizes with little to no forklift or impact to service quality, that is if someone knew about QOS and VLANs. At the time Avaya had their IP Office, Mitel had their ICP/CX line, Nortel had their BCM (albeit the latter two had PC form factors), Cisco had Call Manager Express using app sized phone service along Voice enabled Cisco Routers.
The KSU marketplace may be under demise faster than the PBX cousin with the rise of Voice over IP and the rise of Unified Communications, Cloud or SAAS based telephone systems. With Comcast, Spectrum (old Time Warner/Charter/etc) and Verizon pushing “Business Class” phone systems to customers under fifty users so easily, it has made it even more of a compelling case (for some) to dump their KSUs and go over IP and risk outages, DDOS attacks, errors in “provisioning servers”, etc.
Some of these cloud services are just as minimal to the early 1990s, and often are more handicapped than modern systems of recent years.
What is old is new again.