Or the parent should just take the phone away! If the kid won't listen to it quietly then they should do that thing that I believe is called "parenting". Bring a picture book or something for them if they need to be entertained without the phone.
This was done by my parents when I was a young kid. I wouldn't turn the volume down on my Game Boy on a flight, so my parents took it away from me until I promised to keep the volume down, which I did after that.