The best thing to do is not answer - period. Block the call if you can. But once you answer they know they have a good number. Asking them to put you on their do not call list is a joke. If they were legitimate and you are on the national DNC list they wouldn't call in the first place.
Though I am on the "Do Not Call Register", I went ahead and pressed "1" , just to see hear the person speaking. A ghetto sounding female voice started to speak, but I politely told her that I would like to screw her mother. She hung up.
These people keep calling and leaving a message on my recorder about some sort of free thing for seniors, they will not stop this is so annoying doesn't do any good to be on the DO NOT CALL LIST. I still get this sort of thing very annoying.