We like and use the PortSIP SDK for our softphone development (yes Astiostech’s coming up with one real soon). I downloaded and tried their softphone for Windows, softphone for Windows Mobile version 5 and 6 and i found it absolutely touch (like HTC touchscreens) and look quite similar to HTC’s Manila/Touch interface dialpads etc.

I highly recommend you to use this, sounds great on the earpiece, just like using a normal phone, has buttons to enable speakermode etc…
Works great on WiFi or GSM (3G). Do note, with GSM/EDGE there’s some latency, naturally. So, whenever possible, use 3G/HSDPA
Download here: http://www.portsip.com/downloads.htm