Through my work I recently got a Qtek 9090. Since it got WLAN I have been intrested to try VoIP on it but my first attempts at finiding some suitable software turned out eith no results, atleast not any free software. But yesterday evening I looked around a bit more and found atleast two options:

SJPhone has a Free World Diaplup version for PocketPC

WiFive Phone is another option. The homepage is broken at the moment.

So far I have not had time to try them but will try to do it soon. Will update this entry about the results.