Caller i.d. is obtained, for landline or mobile phone. Caller chooses to not answer unfamiliar phone numbers or suspect identifications (such as “unavailable” or “anonymous”)---but still allows them to ring-through to messaging.
A commercial answering machine, service, or new software app is obtained which instructs the caller to make a human-level determination in order to contact the person/organization: such as “You have reached the phone number of Joe Smith. To speak with him or leave a message, please enter the following sequence: ‘A, 9, 7, X.’” (with the public PIN set and changed by the owner of that phone number whenever he or she wishes.)
All callers willing to have a human make the calls (and enter the publically-proclaimed PIN) can leave messages. All that do not enter the publicly-provided PIN when requested are rejected. The individual customer decides whether or not to “pick up” on clearly-identified emergency or otherwise legal robocalls.