harmonica bug

Etymology
From for the tone that activates or deactivates the device, and  (an electronic eavesdropping device).

Noun

 * 1) An electronic eavesdropping device (bug) that attaches to a landline telephone and is activated or deactivated by a specific tone. When activated, it prevents the telephone from ringing but activates its microphone to transmit to another telephone at a remote location.