The US has 4 mobile network providers. Sprint is useless, Tmobile is okay in certain areas, but really, the 2 with nationwide coverage are ATT and Verizon. In my experience, Verizon coverage is more solid than ATT, but in general these are the 2 networks that provide the most comprehensive coverage.

A natural monopoly exists anywhere there are extremely high infrastructure costs, such as running pipes and wires to every home. The same thing exists with mobile networks, as it requires towers everywhere.