Its not widely known, but like many Nokia smartphones Windows Mobile also supports SIP-based VOIP natively. Of course most carriers would rather not sell you a phone than sell you one with VOIP enabled. XDA-Dev has, as usual,come to the rescue and has released a single cab file that will enable the hidden VOIP capability of your phone.
Simply download VOIP WM6.1 SIP Config and execute the cab, 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.
A real breakthrough for this cab is that on many HTC smartphones the sound finally comes through the earphone instead of the speakerphone.
A full back-up is in order before installation, as some users have reported issues with being unable to uninstall the cab and it affecting other functionality like GPS on their devices. Read more in this thread.