I changed my monthly phone plan on Jan. 23rd. My original billing date was the 2nd. of the month. The plan amount has not changed, it is still $60.00. There was a credit amount of $18.00 for the unused days on the original plan and my billing date was changed to the 23rd. There was, however, a debit amount for $18.00. What is this for? If I am being billed $18.00 for the new plan, then my billing date should remain on the 2nd. If, however, my billing date is being changed to the 23rd., then I should not be billed the $18.00.