This is how the interview happened by accident:

"So at 10:45 on a Saturday morning in late March, we called him on his cellphone. (Don’t ask how we got his number. All we can say is that the White House staff have imperfect control over Trump’s personal communication devices.) The president was at the country club he owns in Bedminster, New Jersey. The number that flashed on his screen was an unfamiliar one, but he answered anyway. ‘Who’s calling?’ he asked.”  The rest is history.