 This indicates that answering machines should stop recording, notifies conference call bridges that a participant has left or removes an abandoned call from a hold queue or interactive voice response menu.The signal may be implemented by removal of battery on the circuit (open switching interval) or by polarity reversal of the tip and ring conductors of the telephone line.An analogue telephone line may also send tones, such as a busy signal, reorder tone, or dial tone, to indicate a call has ended.