The Legend will support disconnect supervision - but if MICS isn't passing it, there's nothing to "supervise" against.

I would handle it like this: Have callers ring the Legend auto attendant as they do, but FORCE the caller to press a digit to get to the receptionist. If the caller dials nothing, play the auto attendant again. If still nothing, disconnect the call. This helps to assure that you have a live body on the line.
Mike