I didn't get that scene.
Teddy called Sam to tell him what she found out (the typewriter), but then she changes her mind in the last second, says everything is fine and hangs up.
She did it because she wanted to face Jack by herself?
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Hi Igorleoni, my impression was that while talking to him she realizes that if Jack is the killer he would come after her, and yes, obviously she wanted to be alone, hence be the bait and face him because she had to know for sure that it was him. However, a la Prizzi's honor she was prepared.
For who would bear the whips and scorns of Hollywood... (;-p)