Please have a look at the suggestion and discussion I’ve posted here:
The idea is to make something that does for VoIP what blacklists and SPF do for email servers:
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!)