Apple only allows FaceTime video calls via WiFi. However, if you have a jailbroken iPhone, you can use it via 3G cellular network too using some Cydia apps.
- FaceBreak: Install it via Cydia. After installing, reboot your iPhone.Only works with FaceTime. Price: $1.49
- My3G: Install it via Cydia/Rock. Several apps are supported and not only FaceTime. It needs Rock app. Price: $3.99
- 3G Unrestrictor: Just like My3G, this tweak makes apps believe that they are on WiFi instead of 3G. Price: $3.99