<p>If its not mentioned in your recommendation letters, some schools inquire from the agencies or organizations about your claim. Even with it being mentioned in your letters, some schools might still call. This usually happens if you are among the last group to be reviewed after a stiff competition, as it happens all the time in competitive schools, and they just need something to weed out the lesser applicant.
Does not happen very often, but it happens, and sometimes just randomly. So be sure to put the correct amount of hours you volunteered and do not exaggerate anything.
Bets of luck to you.</p>