You're completely ignoring that in many rural areas, there are only one or two providers. When I visit my parents in rural northeast Ohio, there are only AT&T a

You're completely ignoring that in many rural areas, there are only one or two providers. When I visit my parents in rural northeast Ohio, there are only AT&T and Alltel (now Verizon) towers. My T-mobile phone roams (free to me) on the AT&T towers and works just fine. The companies can work out roam-exchange agreements among themselves that allow for sharing of infrastructure.