Guirilandia has an awesome post on a common scam here in Barcelona. Definitely check it out.
I've never worked for one of those places here. I worked for one, called Entertel, while I was in college in Lawrence, Kansas in the mid-'80s. Entertel was a boiler room that was careful, apparently, to operate within the limits of the law. For example, when we made a sale, we were supposed to inform the customer that we were going to record them consenting to buy the product, and we had to tell the customer that we were calling from Lawrence.
It was standard boiler-room procedure, though. We were given a script and told to call up people and sell them something called "credit card insurance." The deal was that if someone stole their card the insurance company would pay back all losses. However, in real life, you're only liable for I think fifty bucks on a stolen card--my card was stolen once and the thief ran up 130,000 pesetas on it, and I wasn't liable for anything since I reported the theft to the police--so the product was completely useless.
What happened was that I did pretty well the first day, but the supervisor constantly pushed me to work harder and faster. They didn't try to fire people up with silly stunts, as I've heard boiler-room operators do, but you were supposed to be constantly calling, dozens of calls an hour.
The second day I made a sale to somebody who was obviously pretty dumb, and after she agreed to buy it she asked me, "Now, what did I just buy?" I explained and she decided not to buy it after all. I figured this job was not the right one for me, since I didn't need the money that badly, I didn't like the pressure, and I didn't feel too good about selling junk that people didn't need, basically cheating them, for minimum wage and pie-in-the-sky commissions. Maybe if the thirty pieces of silver had really existed, I might have stuck around despite my ethical qualms, but they didn't and so I didn't. I quit at the end of the day, and got a job in cataloguing down at the university library, which suited me much better, since it was just honest minimum wage for working with books.