At last! I found a GNU GPL licensed software for VoIP, that fits with the Internet of today. It can be found at http://www.openwengo.org. It's distributed in source code form as well and there is an Gentoo ebuild for it, so I did't hesitate to install it. It lacks stability and displays a message that other process is blocking the TCP port (well, there no other application blocking SIP ports). Most luckily, Wengophone has video support! Woohoo, finally!