Ok. There are different kinds of networks. Wifi is the kind of network connection you get from your wireless router. It is fast, and essentially free, because you are already paying your provider. You can also grab a wireless signal at starbucks, and many other public places.
3 g is the data channel associated with cell and smart phones. It is good, because there is almost universal coverage, but it is slow, and it is limited by how much your particular plan will support. It is slow enough that a face time call will not be very enjoyable, so apple limits the app to wifi connections.
The i pad and the i phone both have wifi radios built in, and they are set so that if the device sees a wifi signal, it will use that over 3 g.
So go find a wifi signal, and get the phone on a wifi signal and make your call!