XBox Live communication problem

I have a Xbox Live buddy and for some reason we can't hear each other in games or out of games for that matter.
We can't even hook up and hear each other in private chat.
Thing is other people in the game can hear both of us, We hear everyone else, but we can't hear each other.

We do not have each other muted.
We even removed and readded each other from the buddy list.
I'm baffled. And it's annoying when you can't talk to a teammate. Any Ideas? Solutions?

Support said it is probably because one of us is using "an aggressive NAT-type router".
Their solution is to use a different router. yay
My router is a Linksys WRT54GS. It has the latest firmware. As far as the Nat settings, I looked in the setup. I'm not sure what would be considered the Nat settings. I saw one setting that said something about a NAT filter under the firewall settings, but it was unchecked. I even tried disabling the firewall, while I was online with my bud, but it didn't seem to help. I'm clueless on this aspect of it. I've never even heard of "NAT" before this.

Results say it is "Moderate". What would that mean?

Ok.. the results of my Bud's NAT settings say "open"...
So I assume "moderate" on mine would mean the problem is on my end?
Is there a simple way to "open" my NAT settings? I can't seem to find anything in the router setup.

UPDATE
After a few google searches I resolved the Issue. I ended up having to assign my 360 a static IP and open up a couple of ports. My particular router also had to have the UPnP disabled. I got my NAT open and all is well.. Thanks guys for pointing me in the right direction.