When I lived in Dubai, they used to block VOIP and it was a cat and mouse game. Even though the Government spent millions on blocking the technology, they were always six months behind the curve. Unless the airlines are willing to block VPNs (which will lose them a lot of corporate business), they can't stop VOIP. If people jump from Fring to Nimbuz, to whatever, I suspect that this problem will go on.

The bigger problem that these people will face is that the person in the seat next to them, behind them, or in front of them is likely to snitch them out to the stewardess. Google voice will allow you to text message with your friends. Then there is IMs. I'm not sure that there is anything that compelling to require in flight calls.