Linphone is 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
* Linphone makes use of the SIP protocol, an open standard for internet
  telephony.  You can use Linphone with any SIP VoIP operator, including
  the free SIP audio/video service provided by www.linphone.org
* Linphone is available for desktop computers: Linux, Windows, MacOSX, and
  for mobile phones: Android, iPhone, Blackberry