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.
Glad I looked here, as the caller id only showed the number and unknown name. These Life alert scammers call me every few weeks and change numbers often which helps support the scam theories.