Are the signatures available to the end user? I would love to set up a call screener that only accepts verified calls, as most spam uses spoofed numbers. I'm assuming that the major players implement the protocol at least .. I'm ok if the filter rejects things that aren't real land lines or cell phones.
In general "hosted" services will hide the actual token from the end user, though they may offer either filtering features or the ability to tag calls in the Caller ID based on their signature.
Trunking services designed to feed in to a customer-controlled PBX will usually offer either the same sorts of filtering/tagging or complete passthrough of the token.
