It's certainly possible that someone else accessed his voice-mail assuming it wasn't password protected. Honestly, it's a 50/50 deal. Some won't bother with a password while others will. However, if the voice-mail was accessed from another phone, from what I remember, it wouldn't show up on a cell phone bill outside of the phone bill that the call was made from. People in a general area with cell phone service with AT&T would be calling the same voice-mail number from their cell phones to access any messages.
From what we've all seen, that phone was in use. The question is, who was using it. I only wish I had an answer. If the voice-mail was checked from a phone other than a cell phone, I would think the phone company would know the from number that accessed it, but I'm not totally certain about that. I hope that helps.