If you're giving them some sort of answer (machine, voicemail), they'll keep calling and calling and calling and calling. A live voice is what they're after most. Punch them out by giving them a number with no kind of answer and be patient. It does work eventually.
I received 2 calls both in Spanish, which I don't speak. The first time I hit #1 button. A female voice came on the line speaking Spanish. I said I didn't speak Spanish and they hung up. 2nd call I hit #1 button and male voice was speaking Spanish. I asked if he spoke English and he said he did. He then said he would remove my number so they wouldn't bother me again. I never found out why they called. I'm not sure if this was a good idea or not.