The phone actually worked, and there was someone on the other line talking to Colin Farrell speaking as the caller, but Kiefer Sutherland's voice was added in during post-production.
According to Kiefer Sutherland, Colin Farrell's tears during the confession scene were real. Farrell, who lives with chronic insomnia, was not able to sleep the night prior and he was crying at the mere concept of shooting all day. Luckily for him, he nailed it on the first take - so much so, the crew applauded and wrapped.
Screenwriter Larry Cohen originally pitched the concept of a film that takes place entirely within a phone booth to Alfred Hitchcock in the 1960s. Hitchcock liked the idea, but he and Cohen were unable to figure out a plot reason for keeping the film confined to a booth. Once the idea of a sniper came to Cohen in the late 1990s, he was able to write the script in under a month.
Extras hadn't read the script, so most of their reactions are genuine.
After premiering at TIFF, the movie was set to be released on 15 November 2002. However, after the Beltway sniper attacks in October 2002, 20th Century Fox decided to delay the release to April 2003.