It appears that the first Voip over GSM/Wifi has made its way onto Windows Phone. I haven’t tested the product and it IS only available to use with select service providers. Also it is specifically notated as BETA. But what is interesting is that a SIP softphone app is now on Windows Phone, so we know its possible. And I expect other vendors are some where nearby hatching plans to release their own SIP/VoIP client. (perhaps even an open SIP client?) Here is what MobileVoip says about app:
MobileVoip offers free 3G or WiFi calls for Windows Phone 7, Android, Symbian and iPhone users!
If I am incorrect that this is the first VoIP over 3G/Wifi, please let me know via a comment or @matthewlandis on twitter.
Quality Level: Hobbyist.