There is at the moment no specific information on whether or not Phoenix will support multiplayer modes, and if so, whether over LAN, Internet, or both.

However, in the recent history of Bungie games only one has released without some multiplayer functionality (Oni) and this was not the original intent.

Also, Microsoft's Xbox Live service launches later this fall, so if Phoenix is released sometime after that it could, conceivably, be supported on it.

If Phoenix were to support System Link games, but not Xbox Live (as Halo 1 does) then it might be able to connect over the Internet using one of the various gateways (XboxConnect, Aquaduct, etc).