Cell towers, without mountains/buildings blocking them, reach 10+ miles and airplanes don't fly that high... so you are within range of towers while flying unless you're over the ocean.

However, connecting to a tower that far away requires running the radio at maximum transmission power which absolutely kills your battery. Also the towers reject your phone's attempt to connect because they are programmed to ignore distant connections when they know a dozen other towers are within a few miles of that tower. If you're flying over remote areas where towers will accept any connection you might occasionally get enough signal to call 911 but i likely won't be a usable data connection due to how far away you are.

Wether it shows a connection or not, your phone is still reaching out trying to connect and doing handshakes with towers on the ground.