When A Stranger Calls (1979)

High-schooler Jill Johnson agrees to babysit while a doctor and his wife enjoy a night on the town. After she tucks the children snugly into bed upstairs, phone calls from a stranger start to disrupt her quiet evening of study. At first the unidentified caller says nothing and Jill thinks an acquaintance is playing a prank. Several similar calls have her unnerved enough to call the police when the silent stranger still offers no clue to his identity. The phone calls continue - Finally, an eerie voice on the other end of the line asks ominously "Have you checked the children lately?" Apparently the stranger can see her and follow her movements. She closes all the curtains and turns out the lights believing that the darkness is her only hiding place. By now the police are tracing the origin of the calls and they tell Jill she must stall for time the next time he calls so they can complete the trace. More calls - "Why haven't you checked the children?" Jill, by now frightened nearly out of her wits, screams into the receiver "Leave me alone!" and slams the phone down into its cradle. The next time the phone rings it's the police: "We've traced the calls...they're coming from inside the house!"