this is a little trickier to get working and it's not live, but instead using an alternate server via gamespy to play xbox with. you will only be able to play against others with gamespy arcade/tunnel software and not live people.
Playing xbox live or even using that Gamespy arcade tunneling software that the guy above posted would be literally impossible using dial up. It just simply is not fast enough.
ok it is possible to play a 2 maybe q3 player game of halo2 using tunneling software on dialup but any more and you'll be begging for a quick death. even with 2-3 players you get lag on simple things like getting into a vehicle and weapons changes. supposedly it is possible to use xbox live on dialup using a host computer as a gateway.
actually, you're not on live but on a different server. Kind of like Kali.net used to be. That is why dial-up is allowed. Yes, the gaming experience will suck but technically it is possible. Never know, maybe a few of the games will be tolerable.