ringing current is a high - voltage , low - frequency signal which is sent from a telephone exchange via a pair of wires to ring a bell in a distant telephone . it is becoming common practice for distant telephones to be connected via a carrier connection and the present invention provides a device for conveying a high - voltage low - frequency signal along such a carrier connection . the device comprises a sender part and a receiver part connected together by the carrier connection . the sender part on / off modulates one of the carrier frequencies used in the connection at twice the rate of the low - frequency signal . the receiver part detects the on / off modulation and uses it to connect alternate opposite voltages delivered by a voltage multiplier circuit to a line connected , for example , to a telephone bell .