Unpaid fee

I phoned cell in march to change my debit order details,the said it was changed,now in April ,they still debited from the account that I changed in March ,it was then unpaid ,I did Manual payment on the 21April and I spoke to Thokazani Mabaso ,I told her changed the account in March already ,She confirmed it was not changed ,because I didnot receive debit check mandate ,when she changed it ,she asked me send confirmation letter of the new account and Id ,I sent that ,she confirmed it was changed,now I see on my statement that IAM charged 70 for unpaid debit order,so I'm not paying that it is not my fault as I changed the account.