Comment by Analemma_

Me and most people I know have stopped answering the phone completely if we don't recognize the number, because the ratio of spam to useful calls is so huge. Since this screening renders your use case for anonymous calls completely moot, the benefit of allowing them (very small, in my opinion) has to be weighed against the costs of the current system. Just to pick a random one, political polling is completely fucked at the moment, because so many people don't pick up pollster calls.

Edit: actually the more I think about your comment, the less sense it makes. What information could be gained by an anonymous phone call? Please walk me through this scenario, because I don't see it at all. Who is giving away sensitive information to an anonymous caller that they wouldn't give if there was caller ID?