Poor service delivery

During February I qualified for an upgrade of my mobile contract with MTN. Just before the contract expired, I was called by a MTN representative who offered me a new data contract that would have cost me less than the one that I had. However, at the end of April my account was debited with more than double the original amount of my original contract. The same happened the end of May. I contacted MTN client services three times to try and have the issue resolved. I was told that an agent forgot to cancel the old contract and that it will be cancelled. At my fist call they asked me to call back after 48 hours to check if the issue was resolved. It was not resolved, and I did not receive any credit for the amount that I was overcharged with. The second time I was again told that an issue was logged and that I should call back after the weekend to see if it was resolved. Of course, it was not resolved, and I spend an hour with an agent that was just so unprofessional. In the end she logged another issue. I then called MTN again to cancel my contract and was put through to retentions department. The agent at retentions asked me to call back again today 20 June to find out if my complaint was resolved and that I have received a credit. This morning my bank statement showed me that MTN has indeed credited me with R790 (The amount that I was over charged) I was quite happy until I received an SMS from MTN that stated that they did not receive my payment and that I must pay R790 at the nearest store or I will be disconnected. I tried to call MTN 3 times today. First call was cut off, second call as well and with the third call I was told to call back later because there is a problem with the system. This kind of service is unacceptable. How can MTN pay you back and then threaten to disconnect you because they paid me back what they owe me? What is going on? They should get their systems in order and train agents to do a propper job.