The British government said it was reviewing security procedures after a hoax caller claiming to be the director of Britain's GCHQ eavesdropping agency was put through to Prime Minister David Cameron on Sunday.
In a separate hoax call to GCHQ, also on Sunday, a mobile phone number for its director Robert Hannigan was given out. The government said the number provided is not used for calls involving classified information.
"The Prime Minister ended the call when it became clear it was a hoax. In neither instance was sensitive information disclosed," a spokeswoman for Cameron's office, Number 10 Downing Street, said in a statement.
"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."
All government departments have been now put on alert for such calls, she added.
