There are several ways to play Call of Duty, if you have a device that does not connect to the internet then there are single player options. Also there is a campaign option which is single player and you do not need to be connected to internet.
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Only one person per X-box can play Call of Duty 4 online.
No you can only do one player mode
Only one player can play Call of Duty 3 for Wii.
You can only play online for free with a PS3 or xbox360 or a wii. So make an account go to your call of duty game and go to multiplayer go to online connect and there you go your on online mode
the only cheats that you can unlock in call of duty 4 are the ones from collecting Intel in the single player campaign. Other than that there are no other cheats for call of duty 4.