I recently just created an iPhone app that basically does this. It only allows calls through from your contacts, all other calls go directly to voicemail that you can review later. Most other apps out there attempt to use a blacklist, which is basically useless as it's trivial to recycle phone numbers or just spoof phone numbers. Imagine if social media worked the same way phone calls do, where anyone's effortless contact is treated the same way, regardless of their degree of connection. I think for a lot of people, this is a great solution, and pretty much eliminates the constant distraction of spam calls. https://www.myhumansapp.com