The Royals manager admitted he was taken aback by the quick response from the Manchester United boss and feels it is testament to the 70-year-old’s character.
He told reporters: "I spoke to him in January about something that I needed to address and he was very helpful to me and giving me good advice.
"I spoke to his son and asked him if he wouldn't mind Sir Alex giving me a ring. And, five minutes after that conversation, a private number came up and it was him.
"So, that just tells you the calibre of the man and what he's about."
When asked what their conversation was about, he said: “That's between him and me and obviously just talking to him was good for me.”
McDermott admitted he doesn’t know Ferguson that well but has heard the Scot will give advice to any fledgling managers who ask for it.
"I don't know him particularly well but I just know him from that instance and I know that he's done that with a lot of young managers in the past.
"He does a lot for football and he does a lot for people in the game, and everyone respects that.
"He has done it, been there, seen it," he said. "Everybody's got the ultimate respect for Sir Alex and everything he's achieved."
Reading take on Manchester United on Saturday at the Madejski Stadium.