The Open Phone Abstraction Library (OPAL) is a C++ multi-platform,
multi-protocol library for Fax, Video & Voice over IP and other
networks.

Opal requires ptlib and swig, which are also available at SlackBuilds.org.
Speex from SBo is recommended, even though Opal ships with its own version.
libavcodec, x264, and ffmpeg are optional, but highly recommended, for 
video support.