All I remember are 3 things:
It did not have LOOP START DISCONNECT - that is, if the other party hung up & you didn't (Or had placed the call on hold), the line was busy FOREVER!
-AND- IT did not have a CALENDAR, so, (NOT A BAD THING) you didn't care about Y2K.
-PLUS- on one model, if a call rang in, and you were talking on an extension set to ring, you would here STATIC.
I worked on one earlier this year, it was the first I had seen in over 8 or 10 years, and the client wouldn't even think about upgrading to a Partner.