Linphone has been added to the repository. It is a software phone based on SIP, which is pretty much the standard for VoIP (Voice over IP).
With it, you can call other people that have softphones on any OS (SIP softphones are available for pretty much all operating systems).
If you want to call from your softphone to a regular phone line worldwide, there are a number of places where you can get silly cheap rates. One place is http://www.sipphone.com/
which was started by the linspire & mp3.com guy.
You can call other softphones for free.
It's kind of like skype, but it's based on Open standards. Also, the software is Free Software, and not proprietary like skype. (The makers of skype also made kazaa, which had undisclosed spyware in it...).
apt-get install linphone
or use synaptic.