Can Callkit block all calls except whitelisted contacts?

Is it possible to create a whitelist call blocking app in iOS 11?

For example, I want to block all numbers except those in my contacts list.

It looks like the api only allows you to create a list of numbers to block, not a list of numbers to allow.

https://developer.apple.com/documentation/callkit

from Can Callkit block all calls except whitelisted contacts?