If you are a Linux user and you want to make free or cheap calls through your computer, there are a number of VoIP applications that allow you to do that. There aren’t as many as a Windows user would find, but there are enough of good and well-built applications that come with your Linux installation to allow you to save money and have a nice communication experience through VoIP.
[b]1. Skype for Linux[/b]
Skype is the most popular VoIP application, with nearly half a billion users. The reason why I put Skype on top of my list while it is not primarily a Linux open-source application is that it is backed by a VoIP service. This means that its use is straight-forward for free Internet calls. No need for SIP configurations.
Empathy is the default communication application on many Linux distributions, including the very popular Ubuntu. It supports text, voice and video communication and is interestingly rich in features.
Before Empathy, Egika used to be the default VoIP application on Linux distributions. Ekiga allows you to make voice and video calls over SIP and H.323 protocols.
[b]4. KDE Softphones[/b]
The following softphone applications come with KDE installations and work with SIP and H.323: KPhone, Twinkle and Qutecom.
Asterisk is no simple softphone or end-user VoIP application, but is a very powerful server engine for VoIP management and monitoring.
Nadeen Unuth About.com