Fringland's free mobile VoIP software Fring is something you might want to try out.
Fring can now be used on Windows Mobile 5 and Windows Mobile 6 devices.
According to Fringland, users are able to work with Fring to complete free VoIP calls as well as conduct multiple live chat sessions using Skype, Google Talk, MSN Messenger, and other programs.
Fring's core feature is VoIP calls doable via 3G cellular or WiFi hot spots. Calls can als be made to PSTN phones using SkyeOut or other services.
The Fring Windows Mobile client can be dowloaded here. The software is downloadable to devices via SMS messages.