I have used T-Mobile All throught the US (Pacific Northwest, Houston Gulf, All of California, SLC, Denver and OhioValley to Philidelphia)Europe, China. Because of my phone frequency I had to use a different phone in Japan and Korea.
They where the first to change to GMS here in the states.
Verizon doesn't seem to work many places in eastern NM. When I moved, I had to pay a fortune in monthly fees for zero service until the contract ran out.
I have limited experience with a T-Mobile prepaid in TX, NM, CO, AZ, and southern CA but never had a problem. I think T-Mobile just switches to whatever is available in the area and there are no roaming charges.
About any phone works in the middle of cities, Verizon is the only way to go while traveling. Add a truckers cell phone antenna and use the phone anywhere.