Apple has allowed VoIP over 3G

Apple has made changes to the SDK, allowing applications to make VoIP calls through the operator's cellular 3G networks. Previously, it could only be done via WiFi.
Apple explained the application of the restriction so that users do not overload the cellular networks. It was also spelled out in a contract with AT & T.
Following intervention by the Federal Communications Commission, these restrictions were lifted.

The first such program that appeared in the AppStore was the iCall application