[fsflover]

Technically, you are only asking your software to turn the modem off. It can disobey if it wants, you can't be sure.

[jeffbee]

The reason airplane mode exists is a radio 30000 feet above the ground violates assumptions baked into the terrestrial cellular architecture. If iPhones were regularly flying around with their radios enabled we would have heard about it by now.

[fsflover]

It doesn't have to do this "regularly", only when someone (not you) wants it.

[jeffbee]

How would the baseband get the signal to reenable itself?

[fsflover]

Hypothetically, it could switch on every hour waiting for a command.