Why Cant I Talk With The Players Im Playing With On Mic On Call Of Duty 2 On Xbox 360?

I have the headset and i’m on xbox live Playing call of duty 2…Why cant i tlak with the players while im playing..i was at my cousins house and he was talking with them at the same time…But i can talk with people but jsut sending them a voice chat invatation..but i weant to talk to everyone while im in game play..Let me know how to do that please