linphone (SIP video-phone)

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 Opus, Speex, GSM, G711, ilbc, Theora, H263-1998,
MPEG4, VP8 and snow.

Video support is enabled by default, but can be disabled and thus
negating the need for ffmpeg. To disable support for video just pass
VIDEO=no to the script.

UPNP support is enabled by default, but can be disabled and thus
negating the need for libupnp. To disable support for UPNP just pass
UPNP=no to the script.

Truespeech is disabled by default, but can be enabled.
To enable support for Truespeech just pass TRUESPEECH=yes to the script.

For h264 codec support, install either msx264 or openh264 SlackBuild.