Harassing telemarketers is like yelling at an autistic or a McDonalds employee. They're easy targets, but it does effectively nothing. While yelling at someone with autism will rightly get you snubbed in most circles, yelling at or otherwise harassing telemarketers is not considered a bad thing to do in my various circles.
poor bastards.
Part of the argument for yelling at them is "If they don't want to be yelled at, they shouldn't take such an evil job in the first place". Well there are a few problems with that. First off, not everyone has the luxury to choose jobs based on your code of ethics. The second problem is a sense of scale: in the list of jobs that are evil, telemarketer isn't really high up. The marketing droid who figured that telemarketing would be a great way to make money is way higher. Evil Oil Executive Corporate Overlord is even higher still.
When ever a telemarketer calls me, I always put on my "Who the fuck are you and why are you calling me" voice, that automagically gets them on guard, then I politely tell them that I am not interested and hang up. If they really bug you, ask to be put on their do-not-call list. Again, politely. I mean, I know that telemarketers are annoying as fuck. But the individual calling you isn't singling you out. It is the marketing droid who figured you fit their target demographics that has singled you out instead.
Food for thought.