John wants to know how he can limit the number of calls on his mobile device to only his contact list. Leo says that's called White Listing and since we're now being bombarded with spam and robo calls, that would be a very good idea. Google Voice can do it, but he'd need to get a new number. He can forward his number through it and Google Voice will then filter out unwanted calls, though.
Apple wouldn't do this because they won't allow third party access. But on the iPhone he can set it for Do Not Disturb, and it will block all calls except favorites and those who call twice. He can even set up a whitelist in the settings. So perfect!
On Android, Leo recommends the Truecaller app.