Automation is the preferred option. Many apps are available. I prefer using MacroDroid , since it is free (upto 5 macros) and easy to learn. To understand how MacroDroid works and required settings, my answer here
Your macro would look like this
- Macro : Ringtone for non contacts (name of macro )
Trigger: Call Incoming →Select Contact → Non Contact
Action: Ringtone Comfigure → Select from drop down
Constraints: None ( Macro runs always )
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