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.