Faulty billing and no rectifying

I am a long standing customer with MTN for more than 10 years. I pay my monthly account on time every time. The payment (EFT)made on the 30th of May 2015 apparently does not reflect on MTN's side. I send numerous e mails with the proof of payment to MTN's customer care and was told the account could not be accessed. I had to phone 808 which I did. They could not help me either. I had to email proof to collectionsadmin which I did for a couple of days in a row now. They were supposed to come back to me within 8 hours. After 3 days I still have not heard from them. The worst is MTN keep on adding rent to an account that was paid months ago! This is totally unacceptable!