I have been overcharged on my contract with MTN 4 months in a row and each time I call them they take down the query give you a reference number and there after nothing gets done about it

I have been overcharged on my contract with MTN 4 months in a row and each time I call them they take down the query give you a reference number and there after nothing gets done about it. It is absolutely blatant behaviour from this company,, they refuse point blank to credit where they have overcharged.When you sign a contract you suppose to pay that price but not with MTN . This is absolutely disgusting