This device prevents the phones from ringing on incoming calls. The calling party just hears the phone ringing and ringing, but it's never answered since the phones are silent on the target's end so they have no idea that someone is calling them. Answering machines, Fax machines, and even voice mail - none of these will pick up either. Yet outgoing calls are completely unaffected and go through normally! The target has no idea there's anything wrong, they'll just begin to think that they've become very unpopular as nobody calls them anymore, or even leaves them messages. If installation of this device happens to be on a business line, the potential consequences are even greater. Installs anywhere along the telephone line.