The ding dong is not just for 2 crew operation. In addition to what sebby said the airline i work for is a jet operation and conducts a 'call back' before take off to ensure that all crew are secured for take-off. Some aircraft we have contain up to 4-5 doors up each side which is why a call back is conducted to ensure everybody is ready to go. Once this call back to cabin crew is complete, then the purser/cabin manager/CSM/FSD will confirm with the flight deck that the cabin is secure for take-off.