I still am that this has been turned into a debate about parenting when surely it's about manners.
(Actually now I have typed that I have just glanced up and seen the OP again, and that is more about parenting, but I've started so I'll finish)
I'm sure we all agree that there are certain circumstances where a mobile phone should be switched off (weddings, funerals, theatre, child's nativity play).
And there are times when the phone may well be on but ought to be ignored. Obviously when driving of course, and I am probably stricter than most, I would also say in any circumstance where you were having intercourse (not necessarily sexual!) with one other person (my examples would be: on a date, having sex, a meeting with the teacher, a heart to heart with a friend, even a casual chat at the school gates) Of course you may have been waiting for an important call but it would be the height of rudeness not to say "Do you mind if I get this?" to your companion.
And there are times when an existing phone call should be brought to an end. (my examples would be: child hurting itself badly, someone putting a plate of food they have cooked you in front of you, needing to go to the toilet - unless you are able to do so very quietly, and I would add, when you are about to go on a ride with a child at a theme park.)
Other people have different ideas of course.