Users can enable DND to prevent incoming calls ringing at their telephone. Callers receive a DND tone (as distinct from busy tone) and if they have a display keyset, a DND message is displayed. To prevent an individual extension from enabling DND mode, the "DND deny" parameter in system programming can be set to "Y".

Do not disturb over-ride.
RMP programming (on an extension-by-extension basis) can give a nominated  extension the ability to over-ride DND at called extensions (by dialing suffix digit "3"), the call becoming a normal internal call.