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.

UK, unless you are quiet about it. Though you might not notice as our reaction only tends to be in body language, or for particularly loud speakers (particularly those who are GOING INTO A TUNNEL or otherwise think shouting will help with bad reception) we might tut.

It's considered rude in Belgium, at least when I commuted there by train. People would leave the wagon to take a call. Maybe it's changed in the past few years. There were also signs that it's prohibited to call in the trains.

i’m in the US, if you took a call for more than a few mins a busy metro car I’d tell you to knock it off