Not true, most flights you can have your phone switched on but it is supposed to be put in airplane mode (any electronic devices are) which will shut off all the signals the phone sends and receives (no Bluetooth, wifi, cell reception etc). Devices are supposed to be switched off during take off and landing but can be switched on once the flight is in the air.
They can be on, yes but there will be no service or connection. You are too far away from cell towers so there is no difference when having your phone in "airplane mode" versus being on at 35,000 feet.
I always leave my phone on when flying and never turn it off. I often text before I've even landed (but that's when you are close enough to the earth to have a signal, not when you are nearly 7 miles in the air.)