Express Talk Business VoIP for Pocket PC 4.01
the author of this app says:
Express Talk is software that works like a telephone to let you make calls on your Pocket PC. With Express Talk you can call anyone on the internet who has installed Express Talk (or any other SIP voip softphone). Calls are free. You can also call ordinary telephone numbers anywhere in the world if you sign up to a gateway service. It uses the SIP protocol and is compatible with most major VoIP SIP service providers.