If you're using a PC, there are a number of VOIP(Voice Over Internet Protocol) programs that basically lets you stream data from your microphone to other people's computers. It's kind of like they connected a phone to your internet connection. Although I think the voice quality is a little less than that of a normal phone, but that's likely due to the quality of the microphone and my speakers.
I only have some knowledge of teamspeak, since that's all that I've used and I haven't used it very often. Basically, one person runs a server program to create a virtual server on their machine. Everyone else uses a client program to connect to that server, and then they start streaming from their microphones. You just leave the program up and running in the background while you're playing. I'm using a windower, but if you're not you'd have to start the client program before going into XI.
I have no idea about how to set up the server though, it was always someone else who did that. Aparently there are some compatability issues as well, since my Logitech headphones won't let me stream from my microphone at all, but the microphone might be broken. I can still hear fine, I just can't talk.
I'm not sure what you'd have to do if you're running XI on the PS2, you might just have to do without.