Android, how to send non-contact calls to voicemail?

This should be easy in 2019. I'm sure some of the old geeks on this
list know the answer. All I want is to send every call that isn't on
my contact list to voicemail, while letting all contacts ring through.

The bad solution I find on the web is to create a contact group called
"all," and set it up with its own ringtone. The problem is that groups
are static, and I would have to add every new contact. Feh.

Surely there is a better solution?
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