Skype can be installed on Backtrack and any other Linux system, although there might be performance differences between the one offered by package manager or the Static/Dynamic one offered through the Skype Website. Through the package manager you can setup skype as follows below:
root@evilbox:~# apt-cache search skype
skype - Skype
skytools - Skype database tools - Python parts
and if you don’t have full functionality, download version from the Skype website.