International Revenue Share Fraud (IRSF) (knock-knock.net)

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This is quite the claim (and I'm not saying you are wrong from making it).

I just would have expected some of these institutions to be better.

Hell of an opening on this blog post. Solid write up! Glad honeypots like this exist.

Shades of "KENNEDY SLAIN BY CIA, MAFIA, CASTRO, LBJ, TEAMSTERS, FREEMASONS": https://theonion.com/november-22-1963-1819587981/

Thanks for sharing your interesting research! Can you explain your honeypotting approach further? How are you "presenting" as a SIP relay? What other honeypots or protocols can you detect?

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Could they be residential proxies or does that not work in SIP?

P.S. this is one reason that "pay for email" doesn't work to stop spam.