I just created an iPhone app that uses a whitelist from your contacts like you described. It's available on iOS 11+ and called MyHumans.

I've tested out the "silence unknown callers" feature in the iOS 13 beta and it works very much in a similar way. "Unknown callers" seems to be any call that isn't in your recent calls or in your contacts.

When I made MyHumans (just launched in the last month), I knew there was a risk of something like this appearing in iOS 13, but I'll be adding some more features.