Oh! If it's against the law, surely that will stop annoying phone calls! How did we not think of that easy solution to the problem!?

You can actually sue them if they do call you after you tell them not to for $1200 per violation. Is also illegal for them to call mobile phones. This technology actually cropped up because collection agencies are getting backlash from using robo dialers. I'd argue collection agencies are much more regulated than telemarketers. Google the FDCPA law.