When you say multiple devices on the same extension, can you telll us what kind of call behavior you are looking for?
Do you want a bunch of phones to ring when someone dials a certain number?
IE, do you want to dial "5000" and then have extension 100, 101 and 103 all ring?
Or do you want to have extension "5000" appear on all phones and then be able to put a phone on hold, and then someone pick it up on the later? (this setup being really difficult to do sadly)