I was taking a yoga class at the gym last weekend, and an older woman kept answering her phone during the class. And when I say "answer," I mean, she would actually answer the phone and talk while still on her yoga mat. Briefly, but enough. I got very fed up (especially because no-one around her was saying anything), and finally went over to tell her to go outside. Either she saw me or got the etheric hint (I wasn't sure), but - hurrah! - she finally stepped out of class to have her lengthy conversation.
You would think people would know better than to turn their cellphones on (and answer them!) in a yoga class. But they don't. I actually find older people can sometimes be worse about this. To be fair, sometimes it's because they are worried about the health of a sick spouse. When I taught my chair yoga class, some of the elderly students would inform me that they needed to keep their cellphones on because so-and-so family member might call (and usually this was for something potentially important but not always). So it's not like I don't have compassion for people, but at least my students had the sense to get up and leave and not talk on the phone in the classroom. I also had given them permission to keep the phone nearby - and they had also asked me first. Plus, the class was small, and due to the nature of the class, it tended to be a chatty social hour more than a quiet retreat. In that context, it was not overly disturbing.
But when you are in a large public class, with 40-50 people in the room, it's simply not going to work if even 1/10th of the class is constantly answering phones. I doubt that this particular woman at my gym yoga class asked the teacher first (I'm not even sure she spoke English, since she was talking on the phone in a foreign language). The yoga teacher, unfortunately, did nothing about the disturbances. I'm not even sure she noticed - she herself was busy rattling through instructions rapid-fire in front of the room and was giving off such an energy of "frazzle" that she didn't seem too aware of the students herself.
Fortunately, some yoga studios have implemented a "no cell phone" policy. It takes pressure off the teacher (from being the bad guy) and sets up rules and expectations up front. It's sad that people even have to be told about this but then again, I've seen people show up to yoga class in jeans and even keep their sneakers on in the classroom.
If a yoga studio does not have a no phone policy, I feel the teacher should be clear to students if it comes up and pull a student aside if they are disturbing the class. No, that's not fun, and I'm guessing quite a few yoga teachers have issues with confrontation. But not taking care of the confrontation means you are passing the buck and putting it on your students, and is that fair to them?