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The last time such a phone call was made by the occupant of Number 10 to the Iranian president was more than a decade ago. That was in January 2002 and the man on the phone was Tony Blair.
This is happening now because talks are resuming in Geneva tomorrow over Iran's nuclear programme.
But I think this is also about a wider thaw in relations between Iran and the West - Britain and America included.
President Obama had a phone conversation with President Rouhani at the end of September. That was the first time such a phone call had been made since the Islamic Revolution in 1979.
We have just appointed a Chargé d’Affaires - a non-resident diplomat. We think that will lead to the eventual re-opening of our embassy in Tehran.
And get the irony of this: The Iranian president announced this whole thing on his English-language Twitter feed a whole hour before Downing Street.