I would try to give the benefit of the doubt here, and figure she probably needed to check the messages, perhaps she is a doctor or intern or lawyer, or some profession where she needed to check, that being said she should have had it on vibrate. I would have kindly asked her to turn the sound off and please leave it on vibrate if it was necessary for her to be able to check messages. And that should have been enough to give her the clue that if it was not absolutely necessary to have it on, she should turn it off, tho you could say that as well without being offensive. She could actually have kept it on her mat or next to her mat if it was important for her to see them. And it wouldn't have been as much of a disturbance. People understand when you try to be respectful. I used to teach yoga, so I am really shocked the teacher didn't say anything, my only thought is that this girl told her ahead of time she needed it on, and neither one thought of vibrate or completely off(sound and vibrate)but sitting on her mat so she could check it regularly. I would tell her next time you see her in class. Offering options is a nice way of dealing with it, because people just don't think of them. Offering to help, is better than asking her to turn it off. and if she really doesn't need it on, she will get the hint. I thought of another scenario, I have a sick child so this has happened to me, where my daughter is in the hospital and you are there stressed and worrying day and night. she could have a sick child and need to rush back to the hospital at a moments notice but really needed this time to get away and breathe. Just one of many, many scenarios that could have been at play. I often when I see people doing something crazy or inappropriate try to imagine why they are doing that and give them the benefit of the doubt, because sometimes the person racing down the street crazily passing people is trying to get the hospital before their loved one dies, or sometimes thins we can't even think of. Even if they are not all true it gives me a better, kinder picture of the world. Hope this helps some.