Security procedures at 10 Downing Street and GCHQ are being reviewed after a hoax call was put through to David Cameron.
The security breach occurred after a mobile phone number for GCHQ director Robert Hannigan was disclosed to the caller.
The call to Mr Cameron was made to an official mobile, but the conversation was described as being "quite brief".
The Prime Minister ended the call after it became clear it was a hoax. Downing Street confirmed that no sensitive information was disclosed.
A Government spokesman said: "Following two hoax calls to Government departments today, a notice has gone out to all departments to be on the alert for such calls.
"In the first instance, a call was made to GCHQ which resulted in the disclosure of a mobile phone number for the director.
"The mobile phone number provided is never used for calls involving classified information. In the second instance, a hoax caller claiming to be the GCHQ director was connected to the Prime Minister.
"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."