VoIP is easy to set up. You install a VoIP program, such as Skype, and plug in a set of ear phones and a microphone in your computer and you are ready to make free calls. For non-free calls you first need to buy credits.
With Skype you can always make free calls to other Skype users. Until the end of 2006 they also let you make free calls to regular phones and mobile phones within the United States and Canada.
Besides being free, VoIP calls have better sound quality than calls made with regular phones. Additionally, the data that constitute the call are encrypted so that nobody can listen in.
Skype also allows you to transfer files. There are no limitations on the size of the files being transferred, other than those imposed by the computer's operating system, which can be Linux, Windows, or Mac. Skype's features also include conference calls and instant messaging. Installation packages are provided for various Linux distributions. Some distros, like Mandriva 2006, have it already included.
Another application with Voice over IP functionality is the Gnome software Ekiga, which has full support for the SIP and H.323 protocols. That is, it includes features like call forwarding, call transfer, call hold, instant messaging, and text chat. The latest version has improved audio quality (Wideband codecs), including echo cancellation. Besides the capability of making voice and video calls to other computers, it is possible to make calls to regular telephones and receive calls from regular telephones.
Unlike Skype, Ekiga is a Linux native. It is free, released under the GPL license, and can be downloaded here.