That works until you land in a situation where the calls are coming from a carrier who can't (or won't) do enough about it, but the carrier has enough legitima

That works until you land in a situation where the calls are coming from a carrier who can't (or won't) do enough about it, but the carrier has enough legitimate traffic that it will annoy many of your customers to just shut them out. Remember: All the actual companies or carriers you can identify have nothing to do with the actual scammers. They are intermediaries and the scammers will hide between legitimate calls in one way or other.