I agree being on the phone unless it's an emergency is entirely rude the only problem is those who try to shame people with sarcastic comments etc is that you don't actually know if it's an emergency do you?
Granted of course if someone is twittering on about Tuesday's bingo night they are fair game
However thus was a discussion at work recently as we are on call managers and I am an escalation on call manager for extremely vulnerable people, I wouldn't dream of answering my personal phone while being served but I'm not going to risk harm to someone to save blushes out of rudeness
One of my colleagues had a checkout assistant think herself very clever and smugly and loudly tried shaming her despite when my colleague answered the phone she clearly answered it with "on call"
However to keep confidentiality of course my colleague was effectively saying yes and no with very little information if someone was listening , particularly because this call was frankly about a graphic situation that I promise no-one wanted to here and involved severe injury and how to support staff who had been covered in blood
My colleague had done a 12 hour day and had 15 minutes to grab food before then going back in
That call was extremely serious (to be fair that's why we were talking about this as it was a serious incident debrief)
She didn't get to eat because the woman was sat on her high horse , grimacing and having a good ole laugh trying to embarrass her out of using her phone so my colleague simply walked away
I'm sure that checkout worker was very proud of herself and somewhat bosom hoikingly told her nearest and dearest she had put my colleague in her place
And yes my colleague had apologised and mouthed sorry. She genuinely could not have ignored the call and was trying to eat in a horrendous day, she couldn't explain the details of the call or give any information ,she had answered "on call" so it was clearly not a silly calll , tried to apologise as best she could and wasn't given a chance to explain it was an emergency
But hey manners were more important apparently