I wasn't claiming that this maintenance mode turns on the power to a phone, rather that if the phone is already powered on, the maintenance mode silently and invisibly turns on the phone's microphone, then transmits audio back to the cell tower.
So if you leave your cell powered on, anyone who knows how to activate that maintenance mode can listen to what you are saying, without your knowledge.
your phone already listens for incoming call signal, this would be just another type of signal.