Sandy,
I've been getting "blocked" calls on my cell phone. I would answer and hang up if I didn't hear a response immediately.
DH, who works for AT&T, said give it several seconds, it might be an 800 number that's dialing, they take awhile to answer. So I did, and guess what? It was some shill dialing for dollars...I said I don't contribue over the phone, and she hung up!!

Tell them when they call to take you off of their "do not call" list. It is illegal to call anyone who makes this request.