Here is where you add all the phone lines you have in your house. LinuxMCE supports the regular, old fashioned type of phone line (aka POTS), as well as ISDN, T1 and VOIP. VOIP, for Voice Over IP, is the most flexible and usually the lowest cost. It uses the Internet to place and receive calls, however you can still call regular phone numbers like you always did, and most VOIP providers will still give you a regular incoming number like you have now.
If you have VOIP lines, your LinuxMCE Core will automatically manage your internet connection for you, giving priority to the phone calls so that they have a clear connection.
Even if you have no phone lines at all, you can always call other LinuxMCE users for free. On your Orbiter just choose "Call LinuxMCE User" and dial their LinuxMCE username.
To add a phone line, choose the type of line and then supply the parameters. There are more documents under this one which explain how to configure the various types of phone lines.