I once saw a television commercial advertising Sprint long-distance packages for overseas calling. At the end, they displayed this phone number.
666 was Aleister Crowley's signature; Crowley claimed that in 1904 he received a telepathic communication from an entity named Aiwass, during which he "dictated" the Book of the Law. He subsequently proclaimed that year the start of the next period of human history, the Aeon of Horus.
Someone at Sprint (or their ad agency) has a sense of humor - or should I say had, since the next time I saw the commercial in question, the number had been changed, thus suggesting that the person responsible for it was probably actively encouraged to pursue a new career elsewhere.