I guess the best thing for you to do is have it, rather than as 'Call of Duty', as a non-descript war game. You could insinuate that it is something along the line sof Call of Duty or Medal of Honour, but instead have Generic War Game...
You won't be able to use their footage or sounds (see Cracker Funk's thread for further details) but I guess if you create those elements of the game yourself you should be safe...