[Marge]

Because the billing system only runs once a month and the error didn't come to light until after the bill run this is why you got the letter now so when your next bill drops on your doormat you won't be so surprised

Your bill will probably look like

Feb Calls (credited back)
Mar Calls (charged)
Apr Calls (charged)

btw from what I can remember this only affected certain customers on one stage of the billing cycle