Ok, first, there is an oligopoly of cell phone carriers: you don't actually have enough for it to be a free market. Second, you likely don't want there to be enough real carriers(i.e. with their own towers), for there to be a free market. That would involve massive amounts of wasted infrastructure. Thankfully, third, because of those infrastructure costs, it's unlikely you'd ever see that many cell phone carriers. Cell phone infrastructure isn't quite a natural monopoly[0], but it's close.

[0] http://en.wikipedia.org/wiki/Natural_monopoly