VoIP (Voice over Internet Protocol) is a way of turning an internet-connected device into a telephone. Most applications use SIP to make calls - this is the foremost industry standard, although there are others. Ubuntu's principle application for this is called Ekiga, and provides many features in addition to simple voice, such as video chat. There are several other applications which may suit you better:
- KPhone the definitive KDE desktop voice application
- Twinkle looks good with KDE desktop; no video chat
- Qutecom also fits KDE desktop better; successor to wengophone, but no tie-in to one provider
- Asterisk an advanced user to professional grade telephony server which can route calls between various sources
- Gizmo5 (on Medibuntu) designed primarily for use with a Gizmo phone account, but can be configured to use alternative voice services.
It is possible to install Skype, but there are several reasons why many Ubuntu users prefer not to do this (see SkypeEthics for more):
- unethical business practices - disclosure of IM conversations to the Chinese government leading to imprisonment of Chinese pro-democracy campaigners.
- closed-protocol - Skype users can only call Skype users; SIP users, however, can call anybody on any network.
- closed-source - What's Skype doing to your computer? Who knows? Is it secure? Who knows? All the alternative applications above are open source.