I have two opinions of these people. First, coming from a customer's opinion, I hate these guys because they often call when I'm home alone. I have found the most effective way to deter them is to speak in another language. It freaks them out and they usually hang up. However, looking at it from the telemarketer's stand point, I feel bad for the saps who have to do it, but not enough to not call them not notice things over the phone in Spanish.

I have also discovered that letting my grandma answer the phone is very effective as well. That was by accident though. I felt really bad for that telemarketer, she sounded so confused. XD