automation - Custom ring tone for callers who block their caller ID?

How can I set a custom ringtone for callers who block their caller ID? I don't want the same ringtone for callers who don't block caller ID but are also not one of my contacts (otherwise, I could set a custom ringtone for all of my contacts and change the default ringtone).

Device: Moto X (2nd Generation) with Android 5.1, security patch level 2016-03-01