i cancelled my contract but they kept debit my acc

my mom and I took 2cell phone contracts under 1 package in her name. when the contract term was about to lapse we notified Vodacom to cancel the contract when the contract term lapses. but they debited her account on the 1st month of cancelation we called them and they promised to fix the mistake but it never happened. then the 2nd mnth they debited again, we decided to go to the bank to reverse the amount , then they called to inform us that she owes R1000. now they keep on calling requesting their money.