Phone ringing when dead
Does an iPhone still ring if the phone battery has died? (iPhone 12)
Not likely, since 'the ringer' runs off the main battery.
..While iPhone can work OK with power, w/o Cellular..
Cellular Data is not required to use phone; yet
a Battery, provides Power for entire apparatus.
Maybe the phone isn't fully without battery power
and could be another issue is behind that problem.