Dish is in the process of building a cellular network. While it already covers most of the US population, it only covers a small minority of the US by land area. To supplement its nascent network, Dish has agreements that allow its subscribers to access AT&T or T-Mobile's network.

Dish has been extremely bad about communicating which subscribers have access to which networks. It's my impression that most people that can use Dish's own network can only use one other network (either AT&T or T-Mobile). However, the company is supposed to be trialing a service that can make use of all three networks. That trial service is probably limited to a small set of markets and devices.