I tried this on iOS and it doesn’t work well because it turns off all other notifications.

While calls do get through from your contacts, you don’t get notifications for anything.

I really just want a contacts whitelist, but since that isn’t an option I use nomorobo which is a subscription service that works really well to block spam calls for $2/month. It’s an iOS app, focuses on not uploading your stuff to their servers, and doesn’t collect your call logs.