Take your phone in Taipei and travel 500 miles to the west. Can you still use it? And 500 miles north. Still $20 per month?
There's your answer.
It's part of the answer, but I don't think it's even the most important part.
The real point is that the cost of cell-phone service in each country is really determined by how much people in that country are willing to pay. The richer a country is, the more it pays.
This is not just a random rant against capitalism, it is inherent in how the system works. The two biggest costs are
- licensing of spectrum and
- renting real estate for cell towers.
Both of these costs will naturally grow as a country grows richer, and will be determined by general interactions (auctions, contract renewals, and so on) which naturally push the price to the maximum which is believed by all parties can be extracted from the pool of consumers.
In such environments technical fixes (like releasing more spectrum) may result in better quality service but they will NOT result in cheaper prices, because the price is not being determined by the TECHNICAL cost of service; it is being determined by what the pool of money is that consumers are willing to spend. Most of that money then passes on to those who hold the monopolies in the system, which means the locations for cell towers and the lease of spectrum.
In this respect, the system very much resembles a number of other broken markets --- housing in many cities, higher education, and much of health being prominent examples.