A telephone connection between two subscribers in telephone networks usually contains one or more 2-4-2-wire loops (further referred to as 4-wire loop), as the connection may encounter a mixture of 2- and 4-wire analogue and digital switching and transmission systems, including PBXs. Signal reflections may occur due to the impedance mismatch at both hybrids that terminate a 4-wire loop. A portion of the main signal will thus be reflected at the far end of the 4-wire loop, return to the near end and be reflected again. Therefore the reflected signal will finally arrive at the listener's end some time later than the original signal. The delayed signal is called listener echo.