Ekiga is a H.323 and SIP compatible videoconferencing and VoIP/IP-Telephony application that allows you to make audio and video calls to remote users with H.323 hardware or software (such as Microsoft Netmeeting) as well as SIP endpoints.

It supports all modern videoconferencing features, such as contact roster, presence status, high-quality audio and video codecs, various video resolutions, registering to an LDAP directory, gatekeeper support, making multi-user conference calls using an external MCU, using modern Quicknet telephony cards, and making PC-To-Phone calls.

Ekiga is available in Ubuntu universe repository, just install it in Ubuntu Software Center.

After installation, launch Ekiga softphone under Applications -> Internet:

Homepage: http://ekiga.org/