Why cant i join my friend's party on halo 3?

[J Money]

Me and him both have out NAT types set to open. we can both play online without eachother just fine. but when we try to join eachother it doesnt let us... anyone help please?
It says something like, cannot connect to the party. NAT type is set to open, that is the most flexible type. go to bungie.net/router for more information. says that for us both. And we both have the same map packs.

 

[Nigel C]

What is it saying when you do it exactly? maybe hes blocking u by accident or he may have maps u dont have

 

[dan]

Try doing a private chat together. and on the xbox guide go to chat and IM it might be set to private.
I'm not sure i know what i'm talking about though.
don't get mad at me if iwhat i'm saying is crap.