The singer/songwriter was in bed with her groom-to-be James Taylor when the call came in from an upset Bianca Jagger.
Recalling the odd telephone chat in her new memoir Boys in the Trees, Carly writes, "James and I were in bed, getting ready for the big day, when the phone rang and James answered. It was Bianca Jagger.
"She told James that he shouldn't marry me because her husband and I were having an affair. She muttered some things I couldn't believe she was saying. Then James said, 'I'm sure that's not true. Carly has told me about it and it's not what you think. I trust my wife-to-be. I trust Carly'."
And he was right to trust her, because Simon still insists she never slept with Jagger, even though they had clear sexual chemistry each time they met.
"Having sex would have actually cooled things off," she adds. "If Mick could have his way, it would be Romeo and Juliet tragic. We couldn't have each other."