A device which can be placed near a telephone set on a table or hung on the wall on which the telephone set may be placed to respond to an undesired call such as a nuisance or obscene call. The device comprises a housing containing a battery power speaker which emits a high frequency tone or howl which can be picked up by the transmitter of the telephone set placed in an opening in the housing provided for this purpose. Placing the transmitter in the housing automatically turns on the speaker. In another embodiment the speaker is omitted and the sound connection made directly to the telephone line.

A polarizing D.C. voltage is generated at the capacitance of the ultrasonic transducer and then short-circuited by a parallel-arranged electronic switch controlled by the command signal frequency to be radiated. A direct current stored in an inductance during this short-circuit phase is supplied via a decoupling diode to the electrodes of the ultrasonic transducer upon opening of the switch, and after having been transformed into a voltage. The polarizing D.C. voltage is thus re-generated periodically.

Transistorized electronic circuitry for producing audio-frequency output signals is manually switched to vary circuit constants and effect controlled multi-tone outputs in desired automatic dequence. Battery voltage variations are avoided and tone control is simplified by way of special capacitor connections.