I doubt he ever was able to. Sonny's death notwithstanding, why would Kay have come to Long Beach to give Hagen her letter for Michael if she had heard from Sonny? Her trip to the mall was a result of her not hearing anything about Michael since the Sollozo murder.
Yes, it's one of the plot gaps in the film. But the novel provides more detail:
SPOILERS:In the novel, Kay returns to her parents' home in New Hampshire right after the shooting. About 3-4 weeks later, she's visited by two NYC detectives, who tell her they believe Michael killed Mac and Sol, and ask her if she knows his whereabouts. Kay tells them Michael couldn't have killed the two--
and that they're going to be married. So, I infer that Kay at that point was simply biding her time, believing what Michael had told her before the shooting: that he needed to be with his family in their time of trouble.
Three days later she shows up at the Mall with the letter that Tom refuses to accept. Sonny was alive at that time, but not at the Mall (or not in Kay's presence). Obviously he hadn't found the right time to get in touch with her, or maybe Tom had convinced him to drop it for the reasons discussed above. Mama comes out and tells Kay to forget about Michael. She infers that he really did kill Sol and Mac.
Then, apparently, about two years went by, with Kay calling the Mall on and off, and not getting anywhere. She visits NYC one day, and calls Mama one last time. Mama tells her Michael's home,
and has been home for six months. Kay is stunned and angered, but Mama tells Kay to come to the Mall anyway--to visit
her. That's how Kay and Michael reconnect.
