Pranksters trick UK's top diplomat. Foreign Office not laughing.

(CNN)The British government has opened an investigation into how a notorious duo of Russian pranksters were able to add British Foreign Secretary Boris Johnson to their list of punked politician and celebrities.

A man pretending to be Armenia's new Prime Minister Nikol Pashinyan had an 18-minute conversation with Johnson.
It covered relations with Russia, the poisoning of former Russian spy Sergei Skripal, the Syrian situation, the United States pulling out of the Iran nuclear deal and advice on how to deal with Russian President Vladimir Putin.
    Vladimir Krasnov and Aleksey Stolyarov -- known respectively as "Vovan" and "Lexus" -- have pranked singer Sir Elton John and US Energy Secretary Rick Perry, among others, by pretending to be politicians.
    The duo have denied insinuations that they are backed by the Kremlin or Russian intelligence, but their crank calls tend to target Russia's real or perceived adversaries.
    "Obviously, this shouldn't have happened," a Downing Street spokeswoman said. "An investigation is underway to determine the circumstances around this call and to make sure that this does not happen again."

    The latest prank

    The pair released an audio of the exchange.
    Johnson congratulated the fake Pashinyan on his "remarkable success."
    "If I have a message to Putin, it's that we don't want a Cold War but we do want to see an improvement in the way Russia behaves," Johnson said.
    Asked for his advice on how to handle the Russian President, Johnson said, "the only thing the Russians respond to is determination and firmness -- be very clear with him about your ambitions."
    The two men exchange a chuckle when the prankster jokes that he hopes Putin won't poison him with Novichok, the nerve agent used to poison Skripal and his daughter Yulia in the UK.
    Johnson also said the UK was "pretty sure... like, almost 100% sure" that the Russian government was behind the poisoning of the Skripals, which he described as "a terrible mistake."
    The UK will continue to "tighten the squeeze" the oligarchs who surround Putin, Johnson said, adding "you throw a stone in Kensington and you hit an oligarch."
    Johnson seemed particularly interested in the prankster's claim that Putin said he could influence opposition leader Jeremy Corbyn.
    "I'm sure our intelligence will be listening on this line and will draw the relevant conclusions," Johnson said. "Thank you very much for that interesting tidbit of info."

    'Childish actions'

    The call ended abruptly after the prankster described a new kind of poison and suggested that Johnson should try it.
    "The Foreign Secretary realised it was a hoax, and ended the call. We checked it out and knew immediately it was a prank call," the Foreign Office said in statement. "The use of chemical weapons in Salisbury and Syria, and recent events in Armenia are serious matters."
    Salisbury is the English city where Skripal and his daughters were poisoned.
    "These childish actions show the lack of seriousness of the caller and those behind him," the office added.