If we receive it later than the 10th business day, we still have to provisionally credit - right?
Wrong. If the written confirmation is received later than the 10th business day after the oral notice was given, the provisional credit is not required. There's nothing wrong with giving it anyway, but it's not required.
What if we never receive written confirmation - we still have to credit if an error occurred - correct?
Correct. You have to start and finish your investigation promptly (can't drag your heels waiting for the written confirmation), and in all cases within the 45 or 90 calendar day limits (counted from the day of the first -- oral in this case -- notice of the claim).