How do I assign a custom ring tone to a non-contact list number?

[AC Question]

How assign a custom ring tone to a non-contact list number

I would like to set a custom ring tone in such a way that when I get a call from an unknown number (that is not from my contact list) it will ring the custom ring tone.

This will help me knowing that the caller is not in my contact without looking into the screen.

Thanks
Ashok