Linphone is free and open-source application that provides an internet phone.
DOWNLOAD
Free
Linphone is free and open-source application that provides an internet phone or Voice Over IP phone (VoIP).
With Linphone, you can communicate freely with people over the internet, with voice, video, and text instant messaging. It is available for desktop computers: Linux, Windows, MacOSX, and for mobile phones: Android, iPhone, Blackberry.
DOWNLOAD
Free