Abstract: A ringing device for a PABX comprising an alternating signal source and a direct current source connected to a supply line for supplying a ringing signal to subscriber lines. A centralized off-hook condition detection circuit is connected in series with the supply line and a switch, and in series with each subscriber's line is a local off-hook condition detection circuit and a bidirectional contact. At the time of any off-hook, the switch opens the alternating signal circuit, and the corresponding answering subscriber's line previously connected to the ringing signal supply line is connected to the switching network of the PABX before the switch recloses the circuit for supplying the ringing signal to the other called extensions.

Abstract: Method and apparatus for use in telephone exchanges, such as time division multiplex exchanges, for eliminating the side tone of a telephone station by interruption of a restored input signal to a hybrid circuit thereof. A sampled signal is provided to a storage means having an output signal. The storage means output signal is filtered for reconstruction prior to connection to a reception line on the four-wire side of a hybrid circuit. The restored signal is interrupted during moments wherein a transmission signal, output on the four-wire side of the hybrid circuit, is sampled.