Vodacom contract

I had a data contract with Vodacom 2018-03-03 which ended 2020-03-03 I called vodacom February to cancel the contract end of March since I won't be using their services anymore they said I must call March of which I did and I was told contract cancelled and I must stop the debit order after March because end March it will still going to deduct, I stopped the debit order April, to my surprise I was phoned by vodacom August telling me that I am owing Vodacom R700 and something and I asked for what they said for contract that I have with them I asked which contract because I cancelled my contract in time and for that matter the contract was 24moinths so how come I am owing vodacom? I was told that my name is with lawyers now which will reflect bad on my name whereas it is not my fault that I phoned and cancelled and they did not do their job, I'm so frustrated please help.