Greg S,
Four(or three or whatever)digit dialing on a PBX does not require DID's or Centrex, it's pretty much the primary function of the switch. Think of it like an intercom, with a number to dial instead of a station button to push.
Inside the phone switch (PBX) is a digital switching matrix which knows your phone or your called party's phone, or anybody else's by it's port number.
In the switch programming, each port has information about what kind of phone is connected to it, what features it has installed, what restrictions and permissions it's user has, and finally it's directory number which is usually the last three or four digits of it's listed DID number, but not always.
The fun begins after you dial "9" or "8" or whatever you do to get an outside line, but I'm not going to get into that here. Hope that answers yer question.
jsaxe
