George Arzt, who served as Koch's City Hall press secretary, said Obama arranged to call Wednesday afternoon because the mayor could not talk Tuesday, The New York Times reported.
"The president said, 'I'm just calling to tell you that you are in my thoughts and I am sure you'll be out soon,' " Arzt said. "To which the mayor responded, 'As a result of this phone call, I think I'm getting a better room.'"
Koch is in New York-Presbyterian/Columbia Hospital. He was in surgery Friday, undergoing a quadruple bypass. He is expected to remain in intensive care until next week, followed by a step-down program and three weeks of inpatient rehabilitation.
Koch supported Hillary Clinton during the Democratic primaries last year but campaigned for Obama once he was nominated.