Bettmann; Frank Povolny/Twentieth Century Fox/Sunset Boulevard/Corbis via Getty Images

Per the biography, Kennedy later described Monroe's voice during the conversation as having a "sad" and "ethereal, little-girl-lost quality," which she found to be "disturbing." And according to Jackie: Public, Private, Secret, it remains a mystery how Monroe even obtained the number—the bedroom was the only phone line in the house in which calls weren't intercepted by the Secret Service. 

It was just one month after the phone call that Monroe performed her now-infamous rendition of "Happy Birthday" to JFK during his 45th birthday celebration—from which the First Lady was notably absent.

Following Monroe's death at age 36 that August, Tarborrelli noted to Fox that while Kennedy was "bereft" by Monroe's untimely death, she also always felt the star to be a "disaster waiting to happen," and "someone too vulnerable, too weak to be played with by JFK."

However, he also said she ultimately understood her husband. As he put it to the outlet: "Her main concern was JFK. She knew what it took to be with a very powerful man. And great men like JFK have great flaws."