Call of Duty is a video game based on 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. An expansion pack, Call of Duty: United Offensive, was developed by Gray Matter Interactive with contributions from Pi Studios and produced by Activision. The game follows American and 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). On September 22, 2006, Call of Duty, United Offensive, and Call of Duty 2 were released together as Call of Duty: War Chest for PC. Since November 12, 2007, Call of Duty games have been available for purchase via Valve's content delivery platform Steam.