Rule to Send Unknown Callers to Voicemail?

Looks like there isn’t that specific type of rule in there just yet - the closest thing is to ask them for their name, under the User/Phone tabs:
 

From a programming stand point, that would be somethinthey would program in the ‘Custom Rules’ - but they have the old logic from before - the three segments to look at - caller, number, and/or time.

I would write it up in more detail and post it on their Idea boards - I’ve seen many of them actually added to the core programming.
 

If you go to your Screening, Greeting and Hold Music section and click "blocked calls", there's an option for blocking calls with no caller ID. You'd select the "calls and faxes" option from the dropdown and it will send those calls to a disconnect message.

Not quite the same as your VM, but maybe that's good enough for you. 

Thanks, @Joe Cache and @tak-it1 for jumping in here. 
@Mr. Question Did their recommendations help you? If so, can you mark the one that addresses your use case as “best answer”? This can help other customers who share your use case to get the answer they need!
And if not, please let us know how we can help further. 

We could really use this same feature. 

Hi ​@canyouhearmenow, 
This feature isn’t available at the moment, but you can vote for an existing feature request here: Send unknown callers to voicemail.