I quite enjoy some of these by taking the line with (e.g.) Windows sales people that its a pity you didn't ring last week as I have just placed a major order with one of your competitors.
My son, who is an IT guru, handles the Windows security ones and enjoys winding them up for ages and then after 15 minutes or so says he cant see any windows on screen and he uses Linux .
Usually shuts them up.
I agree with the point already made about using caller display.