The title "L'assassino è al telefono" seems to point at one of the typical codes of the giallo (the anonymous call from the assassin), but the film has more in common with a conspiracy thriller, and finds its inspiration in films like "Les diaboliques" by Henri-Georges Clouzot. An original touch of this rare giallo is its setting in Belgium, more specific in Bruges. There's even a Belgian actor to be found in the cast: Roger Van Hool ("Rendez-vous à Bray") shares the screen with Telly Savalas. Director of photography is Joe D'Amato and the musical score is by Stelvio Cipriani!

"When a stranger calls" earned a rightful place in the pantheon of horror cinema thanks to its opening sequence, an extremely tense example of telephone terror! This suspenseful scene forms a well-rounded sequence of its own, and gives an original structure to this serial killer movie that desacrileges the killer and makes him into a pathetic creature outside of the horror scenes.