I am seriously frustrated with the poor treatment from MTN on my account

I am troubled on my account at MTN: I have a cellphone account for my wife, with MTN, which has been for many years now. I had made an arrangement in October this year to pay for both September and October payments, and I paid R600.00 at the end of October. The line was soft-locked in November, stating that there was an amount outstanding still. I was invoiced R654.44 for the end November, which I paid by EFT on the 28th of November 2018. They still sent a debit of R654.44 onto my bank account, which I reversed because I had already paid. They then soft-locked the phone again. When I inquired, they said it's because of the reversed debit. I stated that I reversed because I had already paid the amount. On the 29th of November when I called MTN Customer Care, they confirmed receipt of the amounts, and confirmed that I already had an extra amount of R340.00 due to be credited back to my account because of the over-payment. I am actually frustrated by the negligence that I feel somebody is displaying at MTN. I feel that whoever it is, he/she is just locking the line, without properly checking what is happening in the account.