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 XMPP (also known as Jabber) is also extremely popular.
SIP or XMPP (Jabber)?
SIP has slightly more emphasis on voice, video and corporate telephony, whereas XMPP originated as a solution for chat and messaging but it's extensible nature allows it to support voice and video. Many people use both SIP and XMPP from a single client.
VoIP and messaging on the Ubuntu desktop
Ubuntu's principal application for this is called Empathy, and provides many features in addition to simple voice, such as video chat.
There are several other end-user applications which you may also consider:
Jitsi is the most extensive and versatile open source VoIP and messaging solution
Ekiga was previously the default VoIP solution in Ubuntu
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
Ubuntu as a VoIP / Real-time communication (RTC) server
To start quickly, the Real-time communications (RTC) quick start guide is highly recommended
repro is a SIP proxy server supporting federated VoIP between Ubuntu servers and other external servers on the public internet
Asterisk an advanced user to professional grade telephony server which can route calls between various sources
Please also see the Debian wiki about Unified Communications as many of the Debian solutions are available on Ubuntu.
Skype
It was previously possible to install Skype, but there are both technical and ethical reasons why you should not (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.
- privacy and advertising: Skype's new owners, Microsoft, are scanning the content of Skype calls and building up a huge database profiling all their users to help in more insidious forms of advertising