Just put a Google Voice line behind a FreePBX or Asterisk and you get all the call filtering you want. You can even make your internal numbers or whatever.

I first found the Tin Can cool, but now seeing their privacy policy, it's definitely nothing for me. I'd just use a normal VoIP cordless phone (e.g. Gigaset makes various models), or even a normal corded phone with a VoIP ATA. Some of them might even have integrated whitelisting, but I didn't check.