6.0 marshmallow - How can I send incoming calls from unknown numbers directly to voicemail?

I have received incoming calls from an unknown number (no caller ID provided). When I answer, I get a recording which is clearly spam. Is there a way to send calls from an unknown number directly to voicemail so that my phone does not ring?

This is very similar to Call blocking with spoken message?, but I do not need a new message to play. Regular voicemail is fine.

I am interested in a solution for stock Android 6.0 Marshmallow. There appears to be the ability to block calls in CyanogenMod, which would be suboptimal, but I cannot find a similar setting in Marshmallow, either in Settings or in the Call application settings. Send blind calls to voicemail?

This is different from blocking a known number, as in How can I reliably screen or block callers on a number-by-number basis?

This is different from How to block private numbers because I do not want to block calls, just send them to voicemail, and this option is not available on stock Marshmallow. The call application allows blocking specific numbers but not unknown numbers.