[UPDATE - I found a partial solution here - http://www.joes.com/intercom/index.html - it's an intercom, not a full PBX.]
I'm looking for links to circuit design for an Arduino-controlled mini-PBX that can use traditional analog phones. I'm looking for something with features like the often-cited Elektor circuit from the early 1990s.
I see lots of schematics and circuit ideas, but Ideally I can find a "telephone shield" that I can plug phones and modems into, then eventually plug that into a phone line for connecting elsewhere, a phone line interface will be easier than trying to get any kind of VOIP stack running on the Arduino, obviously.
FCC registration number shouldn't be required here, as I'm expecting to plug into a VOIP appliance port, not an real analog phone line. I do have about 30 surplus Apple-Cat DAAs if I need to use those.