The Englishman has 19 caps for his country and was part of the international side at Euro 2016, but was not picked in the 23-man squad to go to Russia.
Bertrand faced a relegation battle with the Saints last season but is still convinced that he deserves to be on the England plane this summer.
"The phone rang...it was more shock, silence and reflection," he told the Daily Mail. "The chat wasn't too long. He broke the news, there were some awkward silences and then it was over. I didn't try to convince him he is wrong. That was pointless, it was done, the decision was made. It was nice that he had the respect to call me. I never presume but I was pretty confident I would be on the plane. I had been heavily involved, so it was a surprise. There were no hints during previous meet-ups that I had to change — or that I would be left out."
Stones also admitted that telling his mother the disappointing news was a difficult experience.
"That was hard. She was upset. She was the one always there on the touchline with me as a kid. On those cold Sunday mornings and wet Tuesday nights when I was at Gillingham, we dreamed about World Cups. But we are realistic. She doesn't think I'm [Lionel] Messi!