When a class is large enough, it is possible to see a couple of students hiding in the back and stealthily using their phones beneath their desk.
My solution to this problem is to stop the lecture and just look at them. After a few seconds, the students who are paying attention start to laugh nervously.
As the student with the phone realizes he or she has become the center of attention, they tend to modify their behavior.
Students will modify their behavior more effectively when their own peers signal that what they are doing is inappropriate.
