Hi, folks! It's not that we think the coverage map is proprietary, it's just not our coverage map. Our contract is with Telefonica which is a European based company that piggybacks on AT&T / T-Mobile here in the U.S., so you can look up coverage maps for those companies and that should get you what you're looking for!
Repeating Nick's explanation as a reply to give it more visibility to others who see this thread:
"We use the strongest signal that is available for a specific location - AT&T or TMOB. This is true for any location around the country - there is no geographic split in operator selection.
I'm a fan of https://opensignal.com for looking up coverage information as its reported from users of the networks and can provide a more accurate estimate of the signal strength you can receive in that area."