Mtn does not update details which was given a month ago

Mtn did not update bank details I called the call center on the 05.02 to update my bank details the lady took my details her name is Mandy Mankeni I even followed up by sending her a letter from the bank with the new details and to date it was not changed after noticing mtn did not debit me I called the call center and they advised nothing was done .mtn still debited the old account and it came back unpaid.this should not affect my credit record or any additional charges due to Mtns lazy staff who cannot even update simple details.