AThe Caller admits to Stu that he had been following him for some time. Learning his habits, and his intimate details. So The Caller knew that Stu always stopped at that particular Phone Booth to call Pam at certain points in the week. So the caller planted a Sigma 380 in the overhead light inside the phone booth on one of Stu's regular days where he takes a trip to that booth. Next, The Caller makes his way to the apartment and sets up all his ammunition, and his rifle. Next, The Caller ordered the pizzas for Stu and likely told the delivery guy that if he didn't want them, to bring them to an alternate address (the address where the Caller was). Once Stu refused the pizzas, the delivery guy would bring the pizzas to The Caller and the Caller would likely take him captive until the events of the film unfold. Once The Caller found out the police were on their way to the apartment, he would slash the Pizza guy's throat and grab his secondary rifle (or perhaps his actual rifle and the one seen in the apartment was a decoy) and make his exit.If The Caller truly wanted to get away with it and frame the pizza guy, then the rifle in the apartment would have been the one used and The Caller would have had a back-up. Otherwise, the rifle in the apartment was a decoy and he took his actual rifle with him because he simply wanted to escape the scene. Either theory works and it's up to the viewer to decide how it may have played out. While The Caller revealed he was still alive to Stu, he only revealed himself after Stu had been given a dose of morphine and so Stu may have forgotten all about it or shrugged it off as a dream.