I also got tired of getting these harrassing calls. So I called them back and told them I am on the National Do Not Call List and that if I recieved a call from them one more time I would be reporting them to the FTC. They blamed the no answer on their end on their auto dialer. Figures no one wants to be responsible for something they did wrong these days.
I also got a call and no one left a message and no one answers when I call back, although it stays connected. My cell is on the do-not-call list so if it happens again I'm reporting them...