I've become a heavy user of Google Voice these past few months, ever since I got my invite and was able to set it up. Finally! A local phone number. (I kept my old, out-of-state cell number when I moved to Chicago a few years ago.) I tried to get by with Skype and other solutions for a while, but none of them work as well as Google Voice does.
But what does this have to do with spam? Well, much to my amusement, Google voice has a "report spam" button. If somebody leaves me a voice mail message, and I decided I don't want to hear from that person any more, I can select their voice mail message in my Google Voice inbox and click the "Report Spam" button.
Net effect: That guy (or gal) can't reach me again. Their calls go straight to voice mail, the voice mail goes to my spam folder, and I don't get a new voice mail notification. It makes them darn near invisible. I love it.
And if there's spam blocking, why wouldn't there be authentication? There's another setting I love, where you can force callers to announce themselves. I have it set so this happens only when caller ID isn't present or if caller ID is blocked. The caller has to talk to the phone robot, tell it who they are, and if I pick up the phone, that information is passed to me, and then I'm given the option of answering the call or not answering the call.
As far as authentication technologies go, it's not quite as foolproof or ironclad as DomainKeys Identified Mail (DKIM). Still, it's a very useful feature that helps bring my phone functionality nearer to my email functionality.