Jack spies on Sarah, trying to figure out who she is seeing. He takes her cell phone and calls the numbers in it, saying that he found the phone on the train. Christian Shephard catches him calling and tells him to just let it go.
Jack dials a number while Christian is standing there and Christian's phone rings. Jack wants to know why Christian's number is in Sarah's phone. Again, Christian tells him to just let it go.
Later, Jack sees his father talking and laughing on his cell phone, and follows him to a hotel. In a hotel room, Jack finds his father at a support meeting. He wants to see his father's cell phone and they argue. Jack ends up in jail, where Sarah bails him out.
He sees her with a man and asks if that's him. Sarah says it doesn't matter, it just matters who Jack isn't.