If a number is a DID and programmed on your system it will ring. If the carrier delivers DID 1234 to you and you have it programmed at a speed call that points at an internal number of 6789 it will ring that location. The 3300 routes the DID as though it were an internal number.
Did I clarify this properly?
You could use the Hunt Group, a Ring Group, phantom phone with CFA or call reroute. I prefer the system speed call because it is simple.