I'll do you one better: When they call, sincerely start asking them questions about the identity of their business: location, name, ownership, etc. Usually they

I'll do you one better: When they call, sincerely start asking them questions about the identity of their business: location, name, ownership, etc. Usually they'll hang up. Call them back over and over with these questions. Then after a few calls, explain that you know how to use "VOIP APIs" and that you will be happy to spend $100 to DOS attack their call center. That would be illegal, but typically they're in no position to run to the law. Also, I don't know what a "VOIP API" is or if it's possible to use to DOS attack someone with it. But there's probably an API out there that you could use for that, so the threat seems to be enough.