Downing Street says it is to review security procedures after a hoax caller was put through to Prime Minister David Cameron. 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". Radio DJ A government spokeswoman said a notice has gone out to all departments to be on the alert for hoax calls following the incident. She said: "The prime minister ended the call when it became clear it was a hoax. In neither instance was sensitive information disclosed. "Both GCHQ and Number 10 take security seriously and both are currently reviewing procedures following these hoax calls to ensure that the government learns any lessons from this incident." It was not the first time Downing Street had fallen victim to hoax callers. In 1998, radio DJ Steve Penk, pretending to be the then Conservative leader William Hague, managed to speak to Tony Blair. Four years later, hoaxers overwhelmed the Downing Street switchboard by tricking thousands of people into calling Number 10 asking to speak to "Tony".