Stuart Shepard (Colin Farrell) is an arrogant New York City publicist who has been dating Pamela McFadden (Katie Holmes) behind the back of his wife Kelly (Radha Mitchell). While in Times Square, Stu uses a public phone booth to contact Pam. During the call, he is interrupted by a pizza delivery man who attempts to deliver a free pizza to him, but Stu rudely turns him away. As soon as Stu completes his call, the phone rings. Stu answers; a man on the other end, who knows his name, warns him not to leave the booth, threatening to tell Kelly about Pam.
The caller tells Stu that he has tested two previous individuals who have done wrong deeds in a similar manner, giving each a chance to reveal the truth to those they wronged, but in both cases they refused and were killed. Stu must confess his feelings to both Kelly and Pam to avoid the same fate. To demonstrate the threat, the caller fires a suppressed sniper rifle with pinpoint accuracy. The caller then contacts Pam and connects her to Stu, who admits that he is married.
The booth is approached by three prostitutes demanding to use the phone, but Stu refuses to leave, without revealing his situation. Leon (John Enos III), a pimp, smashes the side of the booth, puts Stu in a headlock and repeatedly punches him while the prostitutes cheer him on. The caller offers to "make him stop" and in Stu's desperation, he accidentally asks for this; the caller shoots Leon dead. The prostitutes immediately blame Stu, accusing him of having a gun, as the police and news crews converge on the location.
NYPD Captain Ed Ramey (Forest Whitaker) seals off the area and negotiates to make Stu leave the booth, but he refuses. Stu tells the caller that there is no way they can incriminate him, but the caller draws his attention to a handgun planted in the roof of the phone booth. As Kelly and Pam both arrive on the scene, the caller demands that Stu tell Kelly the truth, which he does. The caller then orders Stu to choose between Kelly and Pam, and the woman he does not choose will be killed.
Stu secretly uses his cell phone to call Kelly, allowing her to overhear his conversation with the caller; she quietly informs Ramey of this. Meanwhile, Stu continues to confess to everyone that his whole life is a lie, to make himself look more important than he really is. Stu's confession provides sufficient distraction to allow the police to trace the payphone call to a nearby building. Stu warns the caller that the police are on the way, and the caller replies that if he is caught, he will kill Kelly. Panicked, Stu grabs the handgun and leaves the booth, screaming for the sniper to kill him instead. The police fire upon Stu, while a SWAT team breaks into the room that the caller was tracked to, only to find a rifle and a man's corpse.
Stu regains consciousness to find the police fired only rubber bullets at him, stunning but not harming him. Stu and Kelly happily reunite. As the police bring down the body, Stu identifies it as the pizza delivery man from earlier. Stu gets medical treatment at a local ambulance; as he does, the real caller (Kiefer Sutherland) passes by and warns Stu that if his newfound honesty does not last, he will return. The man then disappears into the crowd. Later, the pay phone rings and another man answers.
Colin Farrell as Stuart "Stu" Shepard
Kiefer Sutherland as The Caller
Forest Whitaker as Capt. Ed Ramey
Katie Holmes as Pamela McFadden
Radha Mitchell as Kelly Shepard
Paula Jai Parker as Felicia
Tia Texada as Asia
John Enos III as Leon
Richard T. Jones as Sgt. Jonah Cole
Keith Nobbs as Adam
Josh Pais as Mario
Ben Foster as Big Q (uncredited)
Jared Leto as Bobby (in a deleted scene)