Wikipedia
QuteCom
QuteCom (previously called WengoPhone) is a free-software SIP-compliant VoIP client developed by the QuteCom (previously OpenWengo) community under the GNU General Public License (GPL). It allows users to speak to other users of SIP-compliant VoIP software at no cost. It also allows users to call landlines and cell phones, send SMS and make video calls. None of these functionalities are tied to a particular provider, allowing users to choose among any SIP provider.