Twinkle is a soft phone for your voice over IP 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.
Some of the features offered are call waiting, call hold, 3-way conference call, call transfer, and call reject. It supports STUN or a statically configured public IP address for NAT traversal. When using STUN, it will send keep-alive packets to keep NAT bindings alive.