It might have taken Apple an year an a half to allow VoIP over cellular network but it only took Fring a couple of days to update their app to make use of that. Since yesterday one of the most popular chat clients for the iPhone is capable of doing voice and video calls over 3G networks.
Okay, so those video calls are kinda awkward without a front facing camera but it’s still something. At the very least you will be able to use that unlimited data plan of yours properly and cut down on your phone bill. Here goes a brief video demonstrating the app in action.