When A Stranger Calls (1979) Mystery Thriller Movie

Every babysitter's nightmare becomes real... When a stranger calls. A psychopathic killer terrorizes a babysitter, then returns seven years later to menace her again. Director Fred Walton

Plot Summary - A girl (Carol Kane) is babysitting one night. She keeps getting phone calls from a guy saying "Have you checked the children". (They're upstairs sleeping). The phone calls continue. She gets nervous (and never checks the children). She calls the police. They trace the calls, and they're coming from INSIDE the house...