Twinkle is a softphone for your voice over IP and instant messaging
communcations using the SIP protocol. You can use it for direct IP
phone to IP phone communication or in a network using a SIP proxy
to route your calls and messages.

Twinkle is available for Linux only, and is licensed under the GPL.

Twinkle requires commoncpp2, ccrtp, libsndfile and boost, all
available at SlackBuilds.org.
Optionally, Twinkle also uses libzrtpcpp, speex and ilbc, available
on SlackBuilds too.