Under the Call Routing menu, select Route Plan Report. Enter the first set of digits you want to search against. If you have a 4 digit dial plan, then entering 123 will show any extension in the system between 1230 through 1239.
While you are in there, I would recommend running a list on all Unassigned DNs. This will show you any deleted extension. Click the Delete All Found Items to purge them out.