Debit order date change

On 18 November, I called and changed my debit order date from 1 December to 15 December. I thought all was well. Then, at the beginning of the month I get an automated message saying my account is in arrears. I call in again, the agent tells me the message was sent in error and the date on the system correctly reflects the change to 15 December. Last night I got an automated phone call about the arrears again, and this morning my airtime and data is gone. This is absolutely pathetic service from Vodacom!