Linphone 2.1.0Description by the Software Developer:
"Linphone is a web phone that it let you phone to your friends anywhere in the whole world, freely, simply by using the internet. The cost of the phone call is the cost that you spend connected to the internet.
To call somebody, you must provide to linphone a SIP URL : It is something like email@example.com, where toto is a linux user that runs linphone, and machine.com is the name of a host on a network. If you don't know the machine's name you can specify simply an IP address in dot notation (as 192.0.0.1)."