harmonica bug

noun1

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.

   • Whoever invented the harmonica bug realized that it's nearly impossible to find a bug that isn't operating, so he devised one that can be turned on and off by remote control. — 1975