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.
If you have an iPhone, just go to your phone's recent log and click on the I (information)to the far right of the number. This brings up the option to call it back, add it to your contacts contact.. Instead if you lat swipe down to bottom of the options list the last one is Block Caller. It works. At least until they change numbers ..,