Poor business practice by MTN on my contract cancelation.

I recently canceled my cellphone contract with MTN, they asked me to pay the settlement amount which I did pay at an MTN branch so it reflects immediately. At the end of the month they sent me an invoice with the settlement amount even though I already paid, tried calling them they did not resolve it and now they reflected the amount on my ITC/Credit record as if I did not pay them. This is so unprofessional and they told me it takes 30 days for them to clear it but it did not take me 30 days to put me there.