I've had friends who've been telemarketers because they can't find any other work, and I've read about how awful the job can be. Employees have a script they have to follow no matter how obviously you aren't interested and want to get off the phone. If they don't follow it, they get fired. The telemarketer in this comic didn't respond at all to anything I said, just plowed on with his script (which is, I believe, the thing he's supposed to do), but he allowed me pauses to respond to his questions.
What you do is, once you've pegged them for a telemarketer (about 30 seconds), you either wait for a spot to reply or interrupt them (some don't give you a chance to reply until they've gotten out about a minute of their script, and some won't stop even if you try to interrupt so you have to talk over them) and politely but firmly say that you don't want to waste their time so you're just going to hang up. Then do it. If they start talking again, do not start listening. Once you say you're going to hang up, pull the phone away from your ear and do it. Otherwise they have to keep going and you have to wait for your next chance. This is not a situation you can wiggle out of. You have to take charge of it.
I know some people are okay with being rude to telemarketers right off the bat, but many can't disengage until they get to a breaking point. My way, you can be polite but still get off the phone quickly.
[Telemarketer Who Won't Take a Hint]
Beth: Um... Listen, I know you have a script to follow, but I'd just waste your time, so I'm going to go ahead and hang up. Have a good day!
Beth: I am the QUEEN of shutting down telemarketers nicely.
Alex: You go, honey.