Related link: http://www.engadget.com/entry/6437880577833191/
Engadget has a nice piece on how to use your Pocket PC as a Wi-Fi phone. It uses a Vonage account, so you get a real working phone number. Of course, your Pocket PC needs Wi-Fi to make this work.
PocketPCs and SIP
I'm curious too. My only experience using the PocketPC as a phone came by way of XTen XPro running on an iPaq--a frustrating, almost silly, exercise in novelty. Still, it says something about where wireless and soft-driven technologies are headed.