I have an iPhone 4S on Verizon and have an Exchange 2003 account that syncs mail, contacts, calendar, and reminders. Almost daily or often times more than once per day, my calendar is blank. I do not lose anything else, mail and contacts are fine. The only way I am able to resolve it is to go into Settings -> Mail -> Exchange -> and toggle the On/Off switch for Calendar Syncing. After toggling off and then back on, I get my calendar back within a couple minutes. This also happens to someone else I know with the same phone -- 4S on VZ. However, it does not happen to others. I don't know anyone with the issue on ATT. I am not sure how it could be carrier specific, but I am grasping for a common theme. Ideas?