I know that Windows Mobile supports VOIP natively, but configuring and getting it to work is a pain.
I use SJPhone myself for connecting to my VOIP provider. It hasn't been updated for WM6 and doesn't support high resolution screens, but it mostly works.
You can download it here:
http://www.sjphone.org/preview/SJpho...%28preview%29/
Apparently, there aren't many choices when it comes to generic SIP clients for WM.