Debit order of a canceled contract

i was debited an amount of R682.23 which was R262 extra from what i need to be debited for my phone i called the whole of August sent emails went to banks to send you statements for my refund cause i canceled the sim contract on time cause it was not what i app**** for! Used my airtime the whole month calling you but yall didnt refund me come this month end yall debit me R992.94 which is R572.94 extra from what i need to be debited,the only debit order i have with mtn is R420 for the cellphone contract, so am i needed to call again this whole month to request my refund for both month waste my money on airtime again while yall debiting my money, im currently on maternity leave and its unpaid i make sure i have money for my debits order but uou choose to debit more than its expected making my debit order bounce.