I am at the point where I no longer answer my phone if it isn't one of my contacts. If it is important, they will leave a message. I just press the hang up button on my watch immediately, if it shows an unrecognized number, local or not.
Do Not Disturb + allow calls only from contacts = whitelist of callers.
https://www.imore.com/how-receive-messages-specific-contacts...
There are numerous examples of this: The phone system, the email system, any large IM system that gets beyond a certain size, the physical mail system (reigned in by charges there, though), social networks...
It would be easy if the adversaries were not actively using their intelligence to get around whatever solution you think might work. If all the spam phone calls came from the same number, which never moved or changed, this would be easy. But even before you account for the unauthenticated nature of caller ID that allows spammers to trivially forge any number they'd like, the mere fact that they're willing and able to buy up large blocks of numbers and would switch them around a lot would make this a difficult problem. Stopping intelligent, adaptive adversaries willing to dedicate time and resources to this is a very hard problem.
You can also attack this problem by trying to hit the first clause I gave, but that creates a very different kind of system. And in general, you need some sort of system that you don't have to prewhitelist people, so that your doctor can call you even though you didn't whitelist them, etc.
Anonymous communication is useful, but it has been abused enough that I want to opt-out of it.