Cellular/similar services generally have limits on distance from towers in order to ensure service coverage is not impacted and that it's provided correctly. Round-trip times can be calculated and used to deny service where a device/phone is too far from a tower (whether laterally or vertically), even if it technically has line of sight to the tower and the signal can reach.
I use my phone in the air in my personal airplane and it often works (although tends to be spotty). But at higher altitudes, the service tends to go bye-bye. My hypothetical experience would be Verizon, ATT and Sprint. Generally speaking, those are listed in descending service quality/reliability order from my hypothetical cross country flights.
Van's Aircraft - Media and Community Director
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Building RV-8A since September 2014 (N88VX reserved)
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Flying spam can, helped build and fly a few RV-12s and some other RVs
Hillsboro, Oregon (EAA 105)