The install method is simple. Download this file:
Then, go to Today menu, Programs, Sip Config Tool, input your provider settings and save. Then, go to Today menu, Settings, Today, Items and check "Internet Calling". Then, go to Today menu, Settings, Phone, Internet, and select "Only if cellular is not available" or "whenever available" according to your needs.
Softreset your phone, and after that, connect to a wifi network. You will notice that in your Today screen, the VOIP line has changed from "No service" to "Available". If you want to make a VOIP call, click on this line, and will change to "Selected". All the calls you make from now on will be done using your VOIP line.
Hope it helps you!