Acoustic echo is an inherent problem in the design of speech processing equipment such as telephone network and wireless handsets.  Echo cancellation is a technique to reduce the amount of echo in the voice transmission by estimating the echo picked up on the microphone or line on the transmitting side and subtracts it from the ear piece or handsfree speaker on the receiver side.  Since the exact model of the environment is not known and time-varying, adaptive techniques are typically used to estimate the echo. 
When the far-end signal in the send input speech data is canceled below some threshold level, the residual echo is reduced by a non-linear processor.  The cancellation is typically not perfect for a number of reasons.  First, there may not be sufficient training time for the adaptation.  Second, the far-end residual can at best be canceled to the background noise level.  Typically, the non-linear processor removes this far-end residual as well as the background noise by generating no output signal.  These null periods do not sound natural to the human ear.