Mizu Softphone 

Mizu Softphone is a professional VoIP softphone based on the open standard SIP protocol with an easy to use interface for the Microsoft Windows operating system. With MizuPhone, you can connect to any SIP server on the public Internet or on your local area network. The program will integrate seamlessly with your desktop without disturbing you in your everyday work. While you are not in call or chat, it is listening in the background for incoming calls without any impact on your system performance.