Ignored instruction to change debit date

In the middle of March I changed my debit order date to the 4th of each month. Today they attempted to debit my account. Phoned customer services and spoke to Mabula (useless). She told me debit date was the 7th. When asked why they then tried to debit today, she told me the debit order date was the 7th. I freaked out! I did my bit so why didn't Vodacom do theirs? This has now cost me additional bank charges at the bank which I can ill afford. I took the trouble to change my debit order as I knew my salary wouldn't clear in time for the 1st.