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.
To block: enter their name if visible or just use a name of your choice... and their phone number in your address book. I put an "x" in front of their name like this "xscam", that way all "scam callers" are under "x" in my address book and not all over the place. Once that's done and saved, you can click on their name and you'll see the option to "block" them. That's it. Every time you enter another "xscam" in your address book, you'll be asked to "update contact" or "enter a new contact" ......of course you'll "enter new contact". This has worked well for me. Most of them don't call again, but when they do, the caller ID shows the name I've given them "xscam"...... good luck.