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.
Automation is the preferred option. Many apps are available. I prefer using MacroDroid , since it is free (upto 5 macros) and easy to learn. To understand how MacroDroid works and the settings required for your Android version, see  my answer

Note


This is not tested on Marshmallow  as I am on KitKat
You will need to tweak timings in accordance with voicemail behaviour and "Doze" feature of Marshmallow
Voice mail is used solely for unknown callers
This solution also "mutes" unknown callers (no call ring sound) before diverting them to voicemail to minimise spam nuisance


Your macros would look like this


Unknown caller macro


Trigger: Call incoming &rarr; Select contact &rarr;Unknown Caller

Actions: 


Volume Change &rarr;Ringer (Set to zero on sliding scale- this action will mute ringer for unknown caller)
Call Reject
Wait before next action (set time in seconds per voicemail behaviour so that call is sent to voicemail)


Constraints: None


Other caller macro (to reverse muting of call done for unknown callers)


Trigger: 

Call incoming &rarr; Select contact &rarr;Any number

Action: 

Volume Change &rarr;Ringer (Set to required volume on sliding scale- this action will unmute ringer for callers other than unknown caller)

Constraints: None