MTN bad service and incorrect billings

I have a business account with MTN and downgraded one of the lines that were due for an upgrade without a handset to R400 from R1946.35 but MTN continues to bill me the old amount and ignores my e-mails regarding the matter. If I do not pay it they suspend my whole account and have not passed the credit as they should to the account. The effective date of the new premium was 1st March 2024.