I'm late to the party, sorry. But I use an Android app called Extreme Call Blocker. Nothing gets through that I don't explicitly allow to get through. And in the case of unknown/anonymous callers, they don't even get my voicemail: There's a setting to pick up the phone and immediately hang up for whichever callers you want. It's basically the equivalent of just not feeding the trolls. It only costs a couple of bucks, but it's easily the best five bucks (or whatever) I've ever spent for an app.