Quick Question Regarding Xbox Live

Okay, So ive finally got Xbox Live. I was playing on halo 3, and fifa 08 earlier, although it did not let me talk to anybody, or let me hear anybody whilst playing these games. My account is under 18, as i am 16, and i am currently in my months free trial period. Im assuming that you should be able to talk to the opponent (for example in fifa 08) whilst you are playing with them, if they have a headset.

Any ideas?

Thanks.

Sorry if post was not clear.

Whilst playing halo 3 and fifa 08 on live earlier, I had my headset plugged in, but i could not hear anybody whilst playing.

Im assuming that when playing halo 3, you should be able to hear your teammates at least. Any ideas as to why i can not communicate, or hear?