Lots of luck getting one of these scammers to give you a real callback #. On better than half these robocalls there's no one there even if you do answer it.
The DNC list is a joke. The law has no teeth, and it's not enforced. As you noted, a lot of these originate overseas where U.S. law doesn't apply.
What we need is a method of generating caller ID info that can't be hacked. I've read that there is legislation pending to require phone providers to verify that info is legit, but AFAIK it only applies to cell providers, not landlines. The telcos are fighting this, claiming it's too much work for them to make the necessary changes. TS in my opinion.