MRP ON LINE CELL PHONE PURCHASE

I bought a cell phone on line in May 2017 with MRP. The salesman told me it is going to be 24months contract and I will be paying R89.00 per month. May 2019 I knew it was my last payment. In July R130.00 was deducted from my account, when I phoned to ask them for what was the money for they told me for the monthly airtime they used to send to me a number that I never used which was 064......What shocked me I was supposed to send them an email to remind them to stop the payment in May 2019 which I was not told when I purchased the phone.. Why is it my duty to write to them to stop deducting when finishing paying.