Fring is a VoIP client you may want to check out. According to Fringland Fring can now be used on Windows Mobile 5 and Windows Mobile 6 devices.

Users can wok with Fring to make free VoIP calls and chat with multiple users over Google Talk, Skype, MSN Messenger, and other clients.

Fring uses 3G cellular or WiFi hot spots to deliver VoIP calls, as well as PSTN phones using SkypeOut or similar services.