send non contact calls straight to voicemail

Hi.

Why do we not get the option to send calls from from people who are not in my (Apple) contacts straight to email where we can (optionally) leave a message stating they are not in my Contacts List but they can either go directly to voicemail or if they call twice in a row they will ring through (like do not disturb).

Why?

I have an iPhone 8 running running the latest ioS 11.3 My phone number has apparently made it into the US wide ROBOT CALL data base, because I have recently been plagued with a deluge of robot calls. I am dumfounded that Apple has such a non optional approach to this new problem when this same question has been asked for years.