Well after my positive experiences with a Nokia e65 and the built-in SIP client I have recommended a friend to use VOIP with his XDA Orbit which also has wifi built-in.
However, it all seems much more complicated. So far I have found this:http://forum.xda-developers.com/showthread.php?t=303493&highlight=voip
which seems a complicated way to get WM6 SIP working? otherwise it seems people use a SJPHONE software.
Are these the only options or am I missing something? I am just not sure as I have never done anything with Windows Mobile OS.