Twinkle (software)

Twinkle is a free and open-source app for voice communications over Voice over IP (VoIP) protocol. It is designed for Linux operating systems and uses the Qt toolkit for its graphical user interface. For call signaling it employs the Session Initiation Protocol (SIP). It also features direct IP-to-IP calls. Media streams are transmitted via the Real-time Transport Protocol (RTP) which may be encrypted with the Secure Real-time Transport Protocol (SRTP) and the ZRTP security protocols.
Since version 1.3.2 (September 2008), Twinkle supports message exchange and a "buddy-list" feature for presence notification, showing the online-status of predefined communications partners (provider-support needed).