Ask HN: Why is it so hard to make a contact whitelist on a smartphone?

Why can't I automatically send calls that aren't on my contact list to voicemail on my smartphone? This seems to me like an obvious feature that should be built into Android and iOS. Instead, my options are to (a) trust a third-party app with access to my phone and contacts and have it run in the background 24/7 or (b) write a program to do it myself. Am I missing something? Has anyone else experienced this frustration?