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
Gizmo5 (on Medibuntu) designed primarily for use with a Gizmo phone
account, but can be configured to use alternative voice services.
Check the source for more information and learn more and all about your needs