They just switch to a new different number. Also they can clone a known number, so you think you're getting a call from someone you'd expect to be calling you. I'm thinking of investing in a loud whistle. But then you have to wait for a human to start talking, so make SURE it's not Grandma before you utilize it! (Plus then they mark you as live-prey, and computer doesn't add anything about whistle-abuse,LOL)