What should I do when my students use their phones in class?
Well, we are educating adults - and they should be able to decide what's good for them and what's bad. 

Of course, this philosophy does not work out really well in real classrooms - but some students are using their phones to look up terms I used or check concepts I was teaching online - yes, it really happens! 

What I'm doing is confronting them with the results of such behaviour: I'm telling students, that they failed a test because of their phone usage (not only in class but in general). From time to time I demonstrate that they do not follow the course by taking someone as an example for something who is currently using the phone and they do not recognize we are talking about them. 

But if you take it too serious, you can only lose.