David Cameron says hoax call did not breach security
David Cameron has said a hoax call he received from someone claiming to be taking part in a high level conference call, did not "breach security".
The prime minister revealed he received the call on his Blackberry while out for a walk with his family.
Mr Cameron said he quickly hung up when he realised the caller was not genuine.
He told journalists "these things happened" and "no harm had been done" but that steps would be taken to "weed out" such calls in future.
Downing Street says it is to review security procedures after the hoax caller was put through to Mr Cameron.
Number 10 said the caller claimed to be Robert Hannigan, director of government monitoring agency GCHQ.
Asked about the call at a campaign event in Hampshire, Mr Cameron said these things "do happen from time to time".
He revealed that he was out on a walk in his constituency, carrying his daughter Florence on his back, when he took what he told was a "conference call" involving Mr Hannigan.
He said he was quickly suspicious when the caller said he hoped he had not woken the prime minister up.
"I thought that was strange as it was eleven o'clock in the morning," Mr Cameron said and he then asked who the caller was as he did not recognise the voice.
The caller then said it was a hoax call and the conversation ended. Mr Cameron said there was "no harm done... no national security breached".
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".
The BBC's assistant political editor Norman Smith said the caller was put through to Mr Cameron by the Downing Street switchboard and the prime minister's phone number was not given out.
Analysis: BBC assistant political editor Norman Smith
Incredible as it may seem the prankster managed to obtain mobile numbers for both the head of the GCHQ surveillance centre and the prime minister by bluffing his way past both sets of switchboards.
Having been put through to the GCHQ boss Robert Hannigan, he then rang the Sun newspaper to boast of his exploits. According to The Sun he told them "he'd made monkeys" out of GCHQ despite being high on drink and drugs.
Not satisfied with all that, the hoaxer then succeeded in getting put through to the prime minister. No 10 say Mr Cameron realised "within a matter of seconds" it was a hoax call and put the phone down.
They also say no sensitive information was disclosed - and that they have no reason to believe the caller was anything other than a prankster. Nevertheless in an era of heightened terror alert and anxious political debate over increased surveillance - it would suggest simple human error can be a greater vulnerability.
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."
Tony Porter, the Surveillance Camera Commissioner for England and Wales, told the BBC that hoax calls to GCHQ and Downing Street would cause "some embarrassment" and security procedures were likely to be reviewed.
"Anybody viewing that would have to say it is not only a concern but it must never happen again," he told 5 Live Breakfast.
"I would imagine there is an awful lot of work behind the scenes looking at where that gap was and I imagine there would be a lot of work to eradicate that error, that gap, to make sure it does not happen again."
It was not the first time Downing Street had fallen victim to hoax callers.
In 1998, on radio DJ Steve Penk's show, impressionist Jon Culshaw pretended to be the then Conservative leader William Hague, and managed to speak to Tony Blair.