"Have you checked the children?" You don't have to have seen the 1979 original of "When a Stranger Calls" to be familiar with that ominous taunt; it's a campfire chestnut that's up there with the hook-handed serial killer. But just in case there was anyone out there who didn't already know the payoff revelation, the commercial considerately spells it out, thus saving those viewers who like suspense for their $7.50 their ticket money. A protracted cat-and-mouse game that brings nothing new to the genre ensues. Tweenagers who can't get into "Saw II" or "Final Destination 3" -- and people who have "Memento" syndrome and can't remember the spoiled punchline -- are the target audience. Starring Camilla Belle and Tommy Flanagan. Directed by Simon West. Written by Jake Wade Wall. Produced by John Davis, Wyck Godfrey and Ken Lemberger. A Columbia release. Horror/Thriller. Rated PG-13 for intense terror, violence and some language. Running time: 87 min.