Some of us call it the 2 hour rule, some the "flat tire" rule." As long as you check in at the airport within 2 hours of your original flight, the agent should put you on the next available flight without fees. Calling rez does not justify as checking in at the tkt counter. If the next avail flight is the next day, it is up to the discretion of the agent to charge you or not. Strictly their call.