[Jim S]

Remember also that getting the number is a courtesy to the coaches. Sometimes it's just not possible. ie: the facemask call in the middle of 5 guys trying to tackle the runner. When you don't get it tell em so. Up front. They are the ones with the video tapes. If it matters they can figure it out later.
And, if you haven't already, you will once in a while report the number of the guy who was fouled instead of the guy who committed the foul. IR by the official. (Inadvertant Reporting)