Butt Dialing

I was grading papers last night when my cell phone went off with the "this person isn't in you address book ring". I answer and all I hear is a bunch noise in the background–like in a class working on a project.
I hung up after I didn't get a response and the classroom noise continued.
It was probably one of my former students, because they only have my cell number. I can think of a few that may have had my number on speed dial.

CA