Please have a look at the suggestion and discussion I’ve posted here:
http://robocall.challenge.gov/forum_topics/1560
The idea is to make something that does for VoIP what blacklists and SPF do for email servers:
http://en.wikipedia.org/wiki/Sender_Policy_Framework
This seems like the best place to start this discussion - if Asterisk supports a workable solution, others will follow.
What kind of SPF-like spec would work best for VoIP?
PS: The first group to fully design and/or implement something that could work can compete for a large cash prize on that website.
(If you win, please buy me some pizza!) 