Vent server can be downloaded and installed on your machine for free -- it's got an 8 person limit for the free edition though. I've got the server running on mine. The server admin will setup 'channels' or 'rooms' on their particular server. You use one server, but they can swap on a chat room to room basis, barring that the server admin has set up different rooms, or channels, if you will. Most clients utilize the GSM codec; at least most that I've been apart of -- mine does as well. Anyway, you'll need to forward the appropriate ports for the Vent Servers IP address, if you are behind a router. The default port is 3784 for vent, but can be configured to connect through any port of your choosing.

So, if you've got a group of buddies who play MW2 together, you can setup a free vent server and host yourself plus seven other comrades (for a total of eight users,) for free. Hit me up if you need any help getting a server setup. Good luck!