Some great stuff by Martin Geddes on advanced privacy features for voice networks. He describes a compelling feature set for privacy enabling voice networks, and observes that Skype could do this. Skype is almost there. Their call forwarding feature has no automation capabilities at all, and no API for a third party to add them.
It’s about more than privacy, however. Advanced privacy features could easily be extended to decision making about the relevance of a call. Relevance enabled voice systems would be able to answer questions like "Given the current circumstances, do I want to take this call, at this time?" and act accordingly.