Laptop Magazine has a nice How-to guide on using Skype on PCs but also on mobile windows devices. Skype is the most popular VOIP service. It is free and it works - a winning combination.
"That's where Skype for Pocket PC comes in. However, you'll need a relatively fast PDA or very recent phone to take advantage of this application. Skype for Pocket PC requires a minimum of a 400 MHz processor, built-in Wi-Fi, and Windows Mobile 2003 for Pocket PC or later. "