Many schools have policies that do not allow students to use cellphones during the school day; some even have policies that forbid students to have phones with them in school, at all. But these policies do not necessarily extend to teachers.
Not long ago while traveling on an Amtrak train, the woman seated directly in front of me was using her phone as a speaker so all passengers could here her dual conversation. She wasn't in an office with the door closed, and her call was disruptive to everyone around her.
Cell phones are undermining centuries of date etiquette. The doorbell is becoming extinct, and with it the one opportunity you have to look the kid (friend, date) in the eye, assess his demeanor (i.e., sobriety) and make sure he knows I'm watching.