It seems as though at least for smart phones this could be easily solved through technological means similar to those used for screening email, something along the lines of a spam classification system.
I envision my phone having a white list of known good numbers, which would be allowed to cause my phone to ring like it does now; a black list of known bad numbers, which would be immediately dropped; and a catch-all handler for all other numbers, which would send the call to voice mail for easy manual classification into either of the other categories.
I envision my phone having a white list of known good numbers, which would be allowed to cause my phone to ring like it does now; a black list of known bad numbers, which would be immediately dropped; and a catch-all handler for all other numbers, which would send the call to voice mail for easy manual classification into either of the other categories.