Linphone is a VoIP application for Linux operating system (although it is reported to work also on FreeBSD and OpenBSD). Windows XP is fully supported. It uses the Session Initiation Protocol for communication and is licensed under the GNU General Public License. Linphone uses GTK+ for GUI and on Linux it can be also run as a console-mode application. Of course, even though Linphone is a software tool, in order to use it, you’ll need some basic hardware, such as a headset and a microphone. And if you want to use Linphone as a complete phone system you’ll need an account with a VoIP provider.