Call of Duty is a video game based on the Quake III Arena engine (id Tech 3), and was released on October 29, 2003. The game was developed by Infinity Ward and published by Activision. The game simulates the infantry and combined arms warfare of World War II. Call of Duty was accompanied in September 2004 by an expansion pack, Call of Duty: United Offensive, which was also produced by Activision, but developed by Gray Matter Interactive with contributions from Pi Studios. The game follows the American paratroopers, British paratroopers and the Red army. The Mac OS X version of the game was ported by Aspyr Media. In late 2004, the N-Gage version was developed by Nokia and published by Activision. Other versions were released for PC, including Collector's Edition (with soundtrack and strategy guide), Game of the Year Edition (includes game updates), and the Deluxe Edition (which contains the United Offensive expansion and soundtrack; in Europe the soundtrack was not included). Since November 12, 2007, Call of Duty and its sequels have been available for purchase via Valve's content delivery platform, Steam.