I have very limited knowledge of the Spirit system. This system was made by AT&T in the late 1980s (1987 at the earliest.) I have seen many of these back in the early 2000s and rarely anywhere today. This system was pretty simple, maybe that was their selling point. The phones had no display and supported a couple dozen trunks and 48 terminals. I believe it also didn’t have a native auto attendant or even voice messaging. The phones looked like a step up from phones you would have for the home, but wasn’t in the same level of the Merlin type systems.
Anyways, this YouTube video shows a commercial for the SPIRIT system which explains it simplicity in a dark and deep way.
There is a cousin of ISDN telephone AT&T made and marketed, the 6500 series has resemblance to the SPIRIT phones; could still be seen in some universities or any large Centrex installments.