Skype is a P2P Voice Over IP program.

Skype is a free Voice Over IP program for Linux, Windows and
MacOS. It allows users to communicate with other Skype users
running any of the supported platforms, as well as receiving
and placing calls from and to normal land lines as well as
mobile phones.

Skype needs QT, so you'll have that installed (QT is part of 
the slackware distribution)

Please note that the mirror from which we download skype is not
skype.com. This is due to the fact that skype has removed any
and all support to skype-1.3.x in lieu of skype-1.4.x which is 
QT4 based.