When traveling internationally, it's often a good idea to put the phone in Airplane Mode before departure, activate wi-fi as needed, and leave the phone in Airplane Mode until returning to one's home country where calls/messages/data are charged at reasonable rates.
When on wifi, the phone can still receive notice that a system update must be performed, soon, with no user choice in the matter. It can even download and install this update over wifi. However, as part of the update process, the phone apparently disables Airplane Mode, connecting to a local tower to receive messages and whatever else might have been pending, potentially racking up a big bill in the process, which the user did not agree to or want.
Leaving the phone in a Faraday cage during the update, which might block transmissions to cell towers, would cause the update to fail until it is taken out, because wifi is required for downloading the update.
Why does the phone exit airplane mode during forced updates, and is there any way to avoid that?
Edit, responding to criticism that trying to maintain Airplane Mode during a system update is not a sufficiently clear or concrete objective:
There are multiple reasons why one might want to keep a phone in airplane mode. In addition to the cost factor noted above, if a destination does not have compatible cellular signals, a phone out of Airplane Mode may quickly drain its battery searching for towers, especially if the user doesn't know that the phone takes itself out of airplane mode without notice. In some zones, there are also restrictions on cellular transmissions (example) which do not necessarily apply to low-power wi-fi connectivity, such as on-board some commercial aircraft.
There are multiple different reasons why a person might be in a setting where it's important to maintain Airplane Mode but still wish to use the device for offline and/or Wi-Fi capabilities. This question is about how to do that even when the phone maker is pushing a system update.
I would expect "How do I do this for reason X?" "How do I do that for reason Y?" and "How do I do the same thing for reason Z?" to all be closed as duplicates of this same question.