Lumos Maxima Nox
You cannot only say "Lumos." The digital assistant on your phone needs to be turned on. That would be Siri by default on iOS, and Google Assistant on Android.
Therefore, if we're going technical, iPhone users would need to:
1. Say "Hey Siri" to make Siri active.
2. Say "Lumos" once Siri acknowledges you.
3. Alternatively, you can use the actual spell that real wizards and witches use to turn off their flashlights by saying "Hey Siri" followed by the word "Nox."
Google Assistant for Android performs the same function:
1. "OK Google" will start Google Assistant.
2. Say "Lumos" after being acknowledged.
3. By calling up Google Assistant once more and saying "Nox," you may turn off your flashlight just as on iOS.