Cancelled contract but have been debited

Hi, I had a contract with MTN for 6 years. In 2016 Dec I called to pay off my contract which was already on a month on month basis, the agent gave me quote and I paid it and requested to cancel the contract. I was then put through retention department where they cancelled it. I sent an email and they confirmed it was cancelled. In March 2018 I saw a deduction on my account of R225 and I called to say I cancelled this contract, they said no they thought I was just paying the contract in advanced. My question then is who pays a contract in advance. The lady in retention cut me off and never called me back. The no in question is ********** 997 and to top it all, no calls or data was use, so I am being billed for nothing. Angry