When I get telemarketing calls, I tell them I will get the person they're looking for and gently put the phone on the desk. I put some nice tunes on in the background and go about doing whatever i was doing. Eventually, they get the point and hang up. They can call back as many times as they like and they will get the same response. I find that soon after doing this once or twice, the individual gives up. I didn't have to swear at anyone, or get upset. I just wasted a little of their time until they got the point.