The superimposition creates interfering echoes that must be suppressed as far as possible so that they do not interfere with telephone calls, but at the same time do not give the impression that the transmission line is "dead.
This echo suppression is provided by electronic circuits used in the transmission lines. These components suppress the reflected signals and allow the speech signals to pass from the speaker to the listener. The International Telecommunication Union( ITU) has specified the return loss of echoes, Echo Return Loss (ERL), - which is the ratio of the received signal to the transmitted signal. According to the ITU, the echo return loss must be above 6 dB.