Charged for a closed account.

I sent Mtn an email explaining to them that I needed to settle and close my account, they gave me the settlement amount,I paid it up. I received a message days later them confirming that they have received those funds. I paid it up in April. Last month, in August I received an email from Mtn claiming that I owe them R957.00. I called in, I spoke to 3 different agents who also couldn't understand what was happening, they kept transferring me, until the last agent hung up on me. This is really affecting my credit score.