An open source VoIP and video conferencing application for GNOME.
EditByBSEditor: Ekiga (formely known as GnomeMeeting) is an open source VoIP and video conferencing application for GNOME. Ekiga uses both the H.323 and SIP protocols. It supports many audio and video codecs, and is interoperable with other SIP compliant software and also with Microsoft NetMeeting.

Features:
SIP Compliant
Registrar Support
Possibility to Simultaneously Register to Several Accounts
Proxy Support
Outbound Proxy Support
Call Hold
Call Transfer
Call Forwarding on No Answer, on Busy, Always
Configurable Port Ranges
Instant Messaging
DTMFs Support
Message Waiting Indications Support
enum Support
Transparent NAT Support, Assisted NAT Support (STUN, IP Translation)
SIP re-INVITE support you can free download Ekiga 2.0.11 now.