The warning, sent in an e-mail to AT&T staff, came ahead of a major hackers convention in New York where some of the attendees plan to give a demonstration of "social engineering" techniques — ways of getting information that can be used to break into computer networks from the people who run them.

AT&T workers in past years were tricked into giving out sensitive information over the telephone to people pretending to be other employees or customers, according to the internal AT&T e-mail dated on Thursday.

Recorded telephone calls based on those exchanges have been sold as instructional videos to would-be hackers at the HOPE (Hackers on Planet Earth) conference, the e-mail said.

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