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Sphinx was a monster in Greek mythology devouring those who could not solve her riddle. In VoIP, a new service in the role of Sphinx provides protection against SPIT (Spam over Internet Telephony) by discriminating human callers from botnets. The VoIP Sphinx tool uses audio CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) that are controlled by an anti-SPIT policy mechanism. The design of the Sphinx service has been formally verified for the absence of side-effects in the VoIP services (robustness), as well as for its DoS-resistance. We describe the principles and innovations of Sphinx, together with experimental results from pilot use cases.