Trivia: One of the early signs that Stu is one of the killers is during the scene when Ghostface attacks Sidney in her house. When Ghostface chases Sidney up the stairs to her room, you can just barely hear him gasp out the words "Easy, easy!" The next day, Stu says "Ow! Easy!" in response after Tatum smacks him in the head with her lollipop.
Trivia: The guy playing Sheriff Berk also played a very young police officer in A Nightmare on Elm Street.
Trivia: Matthew Lillard tried out for the role of Stu on a whim. His then-girlfriend was at a different audition in the same building. The casting director saw him and invited him to try.
Answer: Landline phones like the one used in the movie cannot dial out a phone number while the line is active. Considering the killer is continuously calling her, she can't dial out on the phone line. This is referenced in the movie itself when her parents arrive home, attempt to dial 911, and can't due to the line still being active.