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Automation is the preferred option. Many apps are available. I prefer using [MacroDroid](https://play.google.com/store/apps/details?id=com.arlosoft.macrodroid) , since it is free (upto 5 macros) and easy to learn. To understand how MacroDroid works and required **settings**,  my [answer here](http://android.stackexchange.com/a/133394/131553)




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Your macro would look like this

 - **Macro : Ringtone for non contacts** (name of macro )

**Trigger**: *Power Disconnected* → *Call Incoming* →*Select Contact* → *Non Contact*



**Action**: *Ringtone Comfigure* → Select from drop down

**Constraints**: None ( Macro runs always )

Related http://android.stackexchange.com/q/142733/131553