Continuity mistake: When Jill puts the piece of paper with the code for the security system on top of the system in another scene it's gone, and when she returns it's back again.
When A Stranger Calls (2006)
Ending / spoiler
Directed by: Simon West
Starring: Tessa Thompson, Tommy Flanagan, Camilla Belle, Katie Cassidy
Jill finds her friend dead in the bathroom and tries to escape with the two children when the man finally appears and starts to chase her all around the house. During the struggle she also finds the maid dead in the water pond. Then as she's being choked she stabs the man in the hand with a fireplace poker and runs out the door as the police arrive. He's taken away and she is taken to a hospital where she starts having delusions about the guy coming to get her.
Harmin Clair
More mistakes in When A Stranger Calls
Stranger on the phone: What are you wearing?
Jill: Combat boots and a parka you jerk, now who is this?
Question: How would the stranger have known that Jill was going to babysit that night. Did he follow her out there? Or was he just hanging around the area of the house waiting to see if the parents ever went out for a night and left a babysitter with the kids?
Answer: The stranger could not have followed Jill to the house because when Jill gets there, he is already at the house. The front door was open. As to the second part of your question: dumb luck. The stranger was hanging out at the guest house just waiting for the parents to leave.
ChristmasJonesfan
I think he might have been stalking Jill. He could have been following her around her home and school. He targets teenage girls and might have been looking for the right moment to strike. For all we know, he broke into her house when no-one was there and could have found the address and maybe a note reminding Jill that she had to babysit, and then he just would get out there ahead of time to wait for her.