Security procedures are being reviewed at Downing Street after a hoax caller pretending to be the head of GCHQ managed to get through to David Cameron on Sunday.
The government said no sensitive information was given out in the brief call.
GCHQ in Cheltenham is also carrying out a security review after the director of GCHQ, Robert Hannigan's mobile phone number was given out in a separate hoax call.
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 is understood Mr Cameron received the call on an official mobile and ended the conversation when it was clearly a hoax and no sensitive information was disclosed.
The BBC reported that the contact number given out for Mr Hannigan is thought to be for an unclassified phone rather than a secure line.
