MTN incorrect billing and inability to load MTN as a beneficiary on my FNB account

I had a frustrating experience with MTN regarding my debit order, which had been my payment method for over 10 years. The issue was that the amount debited from my account was incorrect compared to the invoices issued by MTN. This occurred for two months after I renewed my contract, and one of their consultants informed me that the retention department had incorrectly processed my renewal.

As a result, I decided that MTN should no longer control my bank account and chose to pay them directly since they were failing to debit the correct amount. I was provided with three account numbers and chose the FNB account, as I am an FNB client. However, when I try to add MTN as a beneficiary, I keep receiving error code 133.

I am now frustrated because MTN's accounts department cannot handle their debits properly, and their FNB business account can't be loaded as a beneficiary on FNB. Can MTN please advise on the correct method I should use to make my payments?