I really liked this solution:

1. I recorded a custom ringtone of pure silence (can do this directly on my phone, so no cables no downloads).
2. added the unidentified phone #s to my contact list, used the names Junk01, Junk02 etc.
3. assigned the silent "ringtone" to all the unidentified numbers.
4. feel smug and superior to the pesky jerks.

Going to do that one myself.

I'm in Canada so I haven't had the identical thing though of course we do have those kinds of scams. My favourite defence: I stopped returning calls to unidentified numbers. It was hard the first time but after that it got easier. If someone honest TRULY needs to reach me, either they will evenutally send a letter or I'll hear about them while chatting with a friend whose number IS in my phone.