MTN not allocating the payment that I am making

I have payed R2500 towards the debt that I have with MTN but they are not allocating the money to my account.I was owing R5031 when I made a payment last month but it did not make any difference ,they are still saying I am owing them R5031 today and my monthly installment is only R518. I reported the matter but the keep on saying they are investigating .They went on and handed over my account to SAYA. I need to know what happened to the R2500 before I continue with the payment but they are not taking the matter serious