Useless mtn

Mtn's customers service is the worst! I have paid my my contract in-store early this month but they keep on calling me to pay my bill. I have already proved the bank statement to proof that I have paid and it also shows on their system but they keep on asking until now for a payment. And if I email them also it takes up to 2 to 3 days for them to respond! And when they respond they ask all your details but I already gave them the details. They need training or need to be fired because they are really useless!!!