I just registered my Windows Mobile 6.1 phone to our SIP based IP PBX for the first time. I'm impressed how nicely the built-in WM6.1 SIP phone integrates into the device when you actually configure it!
To configure the SIP info into your WM phone, you need a windows mobile SIP setup application running on the phone. (Note:this is NOT supported by Microsoft...do it at your own risk!)
Below is the link to the .CAB file to modify your phone:
If you have any question please post a comment below.