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.