Cancelled contract

In 2016 January I took a two years contract with Telkom which was to end in 2018 January. They continued deducting money though they were supposed to stop when it expired. I didn't have a problem hence I was using airtime and their data. in June 2018 I called Telkom to officially cancel and was told it will stop in August 2018. It never stopped.My account will be debited and I reversed everytime. Sometimes they debit twice a month. Now they have handed me to collections