Ventrilo is VOIP (voice chat) software that works great with BRPG, and the free version of Ventrilo allows you to host your own server for up to 8 users. Unfortunately, hosting your own server isn't as easy as it could be, so here are step-by-step instructions that should help.
Usually, the GM will be the one hosting the Ventrilo server.
1. Download Ventrilo from here. You want to get both the Server and Client programs.
2. Install both programs, following the instructions provided here.
3. Find the file called "ventrilo_srv.ini" and open it in TextEdit. Edit the following values as follows:
4. Launch the "Terminal" app and select "New Shell" from the File menu.
5. Type "cd ventrilo" (sans quotes) and press Return.
6. Type "./ventrilo_srv" (sans quotes) and press Return.
The server app is now running. You should be seeing something similar to the image posted below.
7. Launch the Client, configure it the way you want, and log into your server. (I'm glossing over this part, which may warrant its own tutorial; for now, this should help).
Ventrilo uses port 3784 by default, so you may need to set up port-forwarding if you have a router or cable modem. Otherwise, users outside of your local network won't be able to log in. You can use this to run a test to make sure the port is open and Ventrilo can accept external clients.