How can I set the ringtone for "unknown" callers to silent?

Does anyone know how this can be done? I can't find any way to add "Unknown" to my contacts from the call list and I haven't found any other setting that would do it.

Any ideas?

I've been struggling with the same problem. Let me know if you find anything.

I had the same problem also.

There is an app called : "Smart Sleep" by Nexoworks

It can make your phone silent and let the calls only from chosen contacts to ring.... Its not direct answer of what you need but its very flexible. You can also create schedules for weekdays and weekends separately... I am using it and its really a lifesaver. Especially in the night and in the meetings...