Cancellation of contract

I phoned a month before my contract ended to cancel the contract on the last day. I received a reference number for the cancellation. However, my bank account was debited with a higher amount than before on th 1st of the month. I dn't understand this as I cancelled the contract.

I have tried to phone Vodacom Customer Care, but I can't get through. I am overseas and these international calls are costing me a lot of money to cancel a contact that I have already cancelled.