A hoax caller - claiming to be the director of eavesdropping agency GCHQ - was put through to the British Prime Minister.
Downing Street said David Cameron ended the call when it became clear what was happening, and no sensitive information was disclosed.
Meanwhile at GCHQ, a mobile number for director Robert Hannigan was given to the caller.
It is understood that the number given out f was for an unclassified phone, and not one of the secure lines used for sensitive communications.
Procedures are now being reviewed, and a notice has gone out to all UK Government departments warning them to be on alert for such calls.