The first 20 minutes of the film belong to Carol Kane, who sells the premise of a baby sitter terrorized by increasingly disturbing calls from a deranged stalker. She is aided by careful cinematography and exact choices with scoring and audio. The ringing of the phone increases in volume each time, and the camera focuses on the mundane rotary phone for long intervals that are only occasionally punctuated by a ring. The music is tense and ominous, vs the stalker's soft and gentle voice. Carol is small statured and also soft spoken, and it seems as if the large house and surrounding pitch black could swallow her whole. The first act balances all of the elements into a steady escalation in tension and an excellent crescendo.
The second act is less focused. The story picks up seven years later with John Clifford, a private eye who is on the trail of "the caller" aka Curt Duncan. Duncan has escaped a mental institution, and is now off his medications and therapeutic support, and is beginning to unravel. At first, Duncan is problematic, but harmless. He ineptly tries to woo a woman named Tracy who he meets at a bar, and his first attempt earns him a beating from another patron. His efforts become more desperate, however, and soon Tracy fears for her safety. His collapse is aggravated by Clifford, who has no character arc to follow, and is wholly focused and finding and destroying Duncan.
Quite simply, the middle of the movie under-delivers. It looses much of the well crafted tension from the first act and replaces it with an underdeveloped idea: that Duncan is a gentle soul who is not fully aware of the violence he is capable of delivering and is, in effect, a stranger to himself. It's an interesting subplot, but there are problems with the execution. None of the other characters are on Duncan's side, (Tracy feels, at best, pity for him, but that is soon replaced by fear) which just leaves the audience. In 1976, audiences were able to connect to Travis, the dangerous loner from "Taxi Cab Driver." But Travis took his rage out on abusive pimps and human traffickers, not women and children. By comparison, Duncan is hardly a sympathetic character. Finally, not enough time is given to exploring Duncan further or fleshing out this concept in full, and we are soon back to the mechanics that powered the start of the movie.
The third returns to the cat and mouse game between Duncan and Jill, the babysitter from seven years ago, now fully grown. The movie is able to quickly reclaim most (not all) of it's earlier rhythm, and we are suddenly back to a soft, but menacing voice calling from a darkened house to ask: "have you checked on the children?"
Four stars for a well executed premise with a clumsily conceived sub-plot.