One certainty in teaching a college class is that at least 85% of the students will be carrying cell phones (the percentage is sometimes higher). Another certainty is that - from time to time at least - someone's cell phone will ring during the class. There are a few ways of dealing with this.
Mine is to tell the class from the very beginning: if your cell phone rings during class, it should have an entertaining ring tone.
This changes a lot of dynamics in ways that might not be immediately obvious.
First, it changes the rules for students. If their cell phone rings, everyone will listen to hear how good their ringtone is. That poses much different risks than having people notice that they are getting a call during class.
It changes my relationship with the students because I am going to be interested enough to notice their ringtone. It's a fairly non-threatening way of acknowledging shared ground without threatening the us/them line between teachers and students.
And most importantly, it prepares my response when a cell phone goes off. After all, each class only hears my "rule" a few times but I hear it repeatedly. And each time I tell it to a class, I prepare myself to be entertained by the interruption the next time a cell phone goes off while I am speaking.
Cell phones are going to ring no matter how often we asked that they be turned off. That's not something that is likely to change.
What can be changed - with just a quick, repeated reframe - is how we incorporate cell phones into our relationships.