Found a fcc tower map here,
If I zoom out my city to suburbs, I see interesting things. ATT wins with 40 towers. Interestingly, Verizon and Sprint both have only 20 towers each. TMobile has 10 towers, lacking a single tower in the rich quadrant of my town. Apparently, given up the affluent area to Verizon. Yet tmobile looks strong in the University and poorest part of town.
Roaming would be needed, according to the map, servicing in the affluent area, the only people that can afford services.
I trust opensignal more than tmobile map. However many users put booster antennas on rooftops, which messes with the data of on the map.
I assume roaming is hit or miss. Is wifo calling and handoff to real voice calling, as good as on sprint plus rw hacked ROM phones? Tell me they haven’t gone to voip calling over 4g or 3g.