This method would apply to cell phone users.
Verizon would have an option to create a "Unknown Caller Voicemail". The caller would not be able to leave a voicemail.
When a caller that is not on your contact list calls the number, they would hear a message: "You have reached this mailbox because
you are not on my contact list. If you must contact me, leave a text message with your name, who you represent and phone number.
I will call you back".
Those on your contact list would bypass the "Unknown Caller Voicemail".
This would block all robo calls.