Reply To: Why does my son’s Rebbi have a smartphone ?

Teachable moment to show your son how to be dan lekaf zechus. Many people use such devices as their notepad. The rebbe is likely to be checking his notes. Second, if the rebbe is on call for a hotline, such as a halacha or other emergency hotline, presumably with the full consent of his yeshiva, he would be responding to them.