Security procedures at No. 10 and eavesdropping agency GCHQ are being reviewed after a hoax caller was put through to David Cameron's personal mobile phone on Sunday.
The caller claimed to be GCHQ director Robert Hannigan. Cameron ended the call when it became clear it was a hoax, and no sensitive information was disclosed, Downing Street said.
The conversation before the hoax was discovered was said to have been "quite brief".
Earlier, the same hoax caller had rung GCHQ and successfully convinced the intelligence agency to hand over Hannigan's personal mobile number. This mobile phone number is thought to have been for an unclassified phone rather than a secure line used for sensitive communications.
According to The Sun, the caller later phoned the newspaper to declare: "I am off my face on booze and cocaine. I had some spliffs too."
The newspaper said he added: "What's really funny is that GCHQ believed the word of someone so mashed they can hardly string a sentence together."
A government spokeswoman 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 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. 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."