Linphone is an audio and video Internet phone with GTK+ and console interfaces. It uses the SIP protocol, and is compatible with most SIP clients and gateways. It uses various audio and video codecs such as Speex, GSM, G711, ilbc, Theora, H263-1998, MPEG4, and snow. Linphone requires libosip2, libeXosip2 and speex. For video support you'll also need ffmpeg. To enable support for video just pass VIDEO=yes to the script. All depenencies are available from SlackBuilds.org