This new Tele-Data Guard keeps calls private. The Tele-Data Guard alerts the user if someone has picked up an extension, when the Tele-Data Guard is installed on all extensions, the extension you pick up automatically disconnects the others. Specifications Green LED alert light Signals whether line is in use Protects faxes and modems from interruption No batteries required Size 2 1/2 x 1 7/8 x 1 1/4