Now don't laugh, but...
I used to administrate a Definity G3 system at a small hospital. I did everything all the way down to the wiring of individual phone sets and end user education. We were exclusively using 4-wire 7400-series sets (in addition to analogue, of course), and had begun the transition to 2-wire (were those 8400s?) shortly before I left the job.
I have been entertaining (this is the "don't laugh" part) the idea of building a wee little PBX for fun/geek factor. There seems to be a lot of "legacy" Definity equipment floating around that can hardly be given away (7400-series stuff, the 4-wire digital and lowww-density analogue circuit packs, etc).
Would it be feasible to do this? What would I need? Have I lost my mind?
