Downing Street says it is to review security procedures after a hoax caller was put through to Prime Minister David Cameron, the BBC reports.
Number 10 said the caller claimed to be Robert Hannigan, director of government monitoring agency GCHQ.
Mr Cameron ended the call when it became clear it was a hoax and no sensitive information was disclosed.
GCHQ is also conducting a review after Mr Hannigan’s mobile phone number was disclosed during an earlier hoax call.
The contact number given out for the GCHQ head is thought to have been for an unclassified phone rather than one of the secure lines used for sensitive communications.
And although the call to Mr Cameron was made to an official mobile, the conversation was understood to have been “quite brief”.