In flashbacks, Jack is on the phone in his office talking to someone about Sarah. He says, "I met a woman on the train, Sarah, and she dropped her cell phone, and I don't even know her last name, but your number was in her phone."
Christian Shephard walks in and looks at the desk. He wants to know why Jack is dialing all the numbers in Sarah's cell phone.
"Because one of them is him."
Christian hangs up the phone. "You've got to stop this, Jack."
Jack dials again. "Not until I know his name, where he works, where he lives, where they first kissed, I want to know what it is about him--"
Christian's cell phone rings and he opens it. Christian looks caught and Jack looks at him, hanging up the phone. "Why is Sarah calling you, Dad?"
Christian tells him to let it go.