Having lived in both Australia and Japan and visited SK, for me it seems like it’s rude in all those places but only in some places do almost all the people care what strangers think enough to modify their behaviour. Which is not as judgmental a statement as it sounds like, because that care, or some might call it pressure to conform, has upsides and downsides.

Yeah, to clarify my comment, I think it's rude everywhere and I suspect many others generally also, but I've only experienced feeling I couldn't do it/people will absolutely stop you in SK/JP. I've been on trains all through Europe, the UK, almost all of North America and usually hear someone on the phone, but nobody ever says anything.