What's going on in the UK with 121% saturation? 1. Brits travel between competing carriers coverage areas often and must carry multiple phones in order to insure that they can be "reached." 2. It's popular in England to activate lines on your family plan for your unborn children. 3. Cell phone contracts are so hard to get out of that many have lines they no longer use but still pay for. or; 4. How many working cell phones someone has clipped to their belt indicates their suitability for mating. It is part of a complex mating ritual in the UK and is sometimes accompanied but the seldom seen ritual "multiple ring tone" dance. (citations needed )
Seeing as cellular service has a lot to do with the propagation distance of waves, I assume you could add a set of data to that and show that the less dense the population, the higher the nationwide average cost of cellular coverage. So as people per square mile goes down, average cell phone plan cost goes up. This is just a hypothesis, somebody with more time feel free to do the extrapolation That link did not have average cost for either canad or australia, both of which are less population dense than US and which I expect would have higher costs