Require phone manufacturers to provide an optional feature that allows users to record a question to be asked on incoming calls. If the question is answered correctly, the phone will ring, otherwise the phone will not ring and the caller will be told that he/she may leave a message. Users can make the question easy, to screen for live callers, such as "Enter the sum of 2 plus 3." Or a difficult question, to screen for only friends or family, such as "Enter the access number I gave you." Users could change their question at any time, for example at night, limiting rings to only family members who know the answer. Users should also be able to turn this feature off if desired.
With this solution, legal robocallers such as charities and political parties can still leave messages. There is no requirement for users to take live calls. Today users can turn off their ringer, and then all their calls go to an answering machine.
With current technology, the additional cost for a phone with this feature would be small. Phone manufacturers also should still be allowed to make phones without this feature for customers who do not want it.
Rollout of this feature doesn't require changes to the existing phone network, or cooperation of the phone companies. Once phones with this feature begin to be manufactured, interested customers could purchase them immediately. For smart phones, this feature could be implemented most likely with only software changes allowing for very rapid adoption.