Continuity mistake: Towards the beginning of the movie, the Bell Atlantic sticker on the phone receiver is faded, but it eventually turns into a brand new sticker. (00:27:30 - 00:31:15)
Continuity mistake: In the scene where the pimp starts shaking the phone booth, ad cards and Stu's wedding ring are visible on top of the phone. After the "shaking" the ad cards and ring are gone. A few minutes later the ad cards are back, mixed in with the broken glass. (00:25:15)
Continuity mistake: Right before the police arrive for the first time, Stu is shot in the ear and reaches up and starts to hold his ear. The camera angle changes to a wide view and both of his hands are now on the shelf in the phone booth.
Continuity mistake: Several times during the movie the police cars forming the perimeter around Stu appear to be closer than they were before. When Captain Ramey first approaches Stu, he has to walk a distance of about half a block. Later in the film he seems to be walking only a few feet.
Continuity mistake: You can see the amount of glass near the cabin changing its size repeatedly during the film.
Continuity mistake: Stu takes off his wedding ring as soon as he gets in the phone booth. However, in one shot where the camera is panning away from him (just after the police call his "lawyer"), he is wearing his wedding ring again. When the shot pans in, it has disappeared. (00:11:45)
Continuity mistake: Right before Stu comes clean and confesses his crimes, Kelly gets out of the police car and you can see two police officers in front of her. The camera angle changes to a front view and there is now only one police officer there.
Answer: The pizza man is intended by the Caller to be a decoy so he can get away. The Caller sends the pizza man to the phone booth so that Stu will identify him later in the apartment, giving the police a convenient answer to the mystery while the real Caller escapes. In fact, at the end of the movie, the Caller comes to visit Stu - obviously he's not the dead pizza delivery man.
Phoenix