Isn't this what Google is trying to attempt with Google Voice and using SPAM filtration on those calls? It would seem that a company could greatly benefit by somehow acting as an intermediary and allowing users to rate a call as spam.

The issue is that like SPAM in the e-mail days, the technology isn't the problem, but rather the adoption of a filtration method. Adoption will happen when this problem becomes a big enough issue for consumers to switch to another provider (which is already difficult already).