To block unknown callers on an Android phone, you can use the Block unknown callers feature in the Settings menu:
Open the Phone app
Tap the three dots at the top of the screen
Select Settings
Tap Block numbers
Toggle the Block unknown callers button to green
I did that RB.....no joy...at least not as much as I should have 😏
Far as I can tell, blocking “unknown” doesn’t block any call that doesn’t come in as “Unknown.” Which is none of them. I get plenty of spam, and always block AFTER the call, but even that doesn’t always work.
And yes I do have block unknown on in the settings.
There should be (probably isn’t) a setting by which you can block anyone who’s not in your contacts.