They Call, then Hang Up so you call them back to see who it is. Once you do that you can not add them to the "Do Not Call Registry" because YOU called THEM. It's a new scam way of Calling so they can't get put on the List.
Same thing, they called and hung up after 2-3 rings. I called them back from a number that originates behind a switch so it wasn't from my cell phone (don't want them confirming my number)
Same message "Hi, this is (Persons name) with Reward Redemption...." offering you a $100 gift card. They ask you to press 1 to receive your reward or 2 to be added to the do not call list ... each option goes to a message that says "this number is no longer in use" ... so clearly they are collecting your number as a live number if you call them back. For many year now I have the rule that I do not answer my phone if its a number I do not know. If someone wants to talk to me they'll leave a message explaining who they are. If they don't have the common decency to do that they are not work talking to anyway :-)