MTN insists on debiting old account even though I updated my account with new account number

Good day

I have a contract with MTN where they give me 2 gig data and 50 minutes and I pay R258 per month. About 4 months ago I changed my banking details from standard bank to investec and I called in MTN To change my details accordingly, but MTN insist on using the wrong account (old account) each and every month . My salary goes to the new account so the old account doesn’t have money , and now I’m forced not to close my old account because of MTN , if I close it my MTN account will bounce, I’m forced to keep my standard bank account all because MTN insist on debiting from that account instead on my new account.

I raised this with MTN last month and prudence ntuli from MTN told me they do have my new investec account and they even shared the details with me but somehow they are still debitting the old standard bank account . Prudence ntuli is the agent who’s responded to my query about MTN debiting the wrong account , she apologized but I see the October debit order still came off standard bank account.

It’s obvious MTN refuses to respect my wishes to use my new account for reason I don’t know so I need help, they are costing me money , I’m forced to keep my standard account because I don’t want to default on a payment and have bad credit record all because they insist on using a wrong account.

I have even written a letter to Icasa about this incompetence