Hi,
Getting accurate information about MNO geographical coverage from the MNOs is, simply put, impossible. They all love to talk about the percent of the population covered by their networks, but never talk about the percentage of the land area they cover. I’ll add that 63% of the US population lives in cities and those cities comprise 3.5% of the US land area. Earlier this month the FCC determined that T-Mobile, US Cellular, and Verizon lied about the geographical coverage of the their 4G networks.
There is at least one group trying to provide accurate coverage information: opensignal.com. They have created an app for crowd sourcing coverage information. I have no position on the accuracy of their coverage maps or their goals.