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 disable support for video just pass
VIDEO=no to the script.
All depenencies are available from SlackBuilds.org