What is the coverage area for BMW Assist in the US? IOW which cell phone carrier(s) does BMW Assist use? Does the system roam to other carriers? What areas in the US does BMW Assist not work because of lack of cellular coverage?

In the Western US there are large areas with only one cellular provider and some areas (e.g. Big Bend National Park) with no cellular coverage at all. We are planing to travel in the west and would like to understand just where we might be 'out of range' for BMW Assist. I ask here because I have been unable to get a straight answer from my dealer or two different BMW Assist 'operators' when we set up BMW Assist*. Perhaps others have been luckier.

Funf Dreisig

* Both my CA and the BMW Assist 'operators' told us that BMW Assist uses Verizon. But we were at home when we setup BMW Assist. Verizon doesn't work at our house, because the towers are blocked by the hills. So either the X5 has a honkin' good antenna or BMW Assist uses/roams to AT&T. We get 5 bars with AT&T at our house.