Why are videos call and voice calls prohibited on airplanes?
Voice/video calls have not been approved by the FAA, and no one is particularly interested in having them approved, due to the frustration it would cause amongst passengers, having to listen to a multitude of conversations in a confined space, throughout the flight. Not to mention the fact that it would clog the limited bandwidth if even a sizable chunk of the passengers wanted to do it. 

Virgin comes right out and says it with something along the lines of "For the safety and comfort of our guests, VoIP and Video Chat is also prohibited".