This device allows you to place the other party on false " hold" but you'll still be able to hear any conversation on the other line. Imagine the potential of this device, for example, maybe you are finishing up a business deal; give them your final offer; now tell them you have another call coming in and place them on "false hold"; listen to what they have to say; now remove the "hold" and use the inside information to your advantage. This is just one example but the possible scenarios are unlimited. The device connects between your phone's handset and base and can be used on multi-line phone systems.