I have been trying to set up a VoIP client on my Windows Mobile phone for a long time, starting with the HTC Wizard which I could never get to work.
I have finally managed to get a VoIP client with a really nice interface working on several phones that I have tried it on.
I am using ViaTalk as my provider but it should work with any VoIP provider.
It works great (with a slight delay, nothing that I can't live with) on an unlocked HTC Sprint Touch Pro 2, a T-Mobile HD2, a HTC Diamond and an HTC Fuze.
The application is by NCH software out of Australia and it is free to use, or if you want to unlock all of the features of the application, download and register the business edition.
Here is the URL for the download:
Or if you would like, PM me and I will be happy to send it to you as an attachment to an email.
Only known issue is when the phone goes in to power saver mode it drops the WiFi connection but that should be an easy fix, right now I am simply pressing the power button and it comes right back and the connection is still alive.