Over charged not according to the contract

I have a contract with Telkom which i took in October 2018. i bought an Iphone 7plus and my contract simply states that i will pay R500 every month for 24 months, but they only deducted that R500 For about 3 or 4 months then the rest till today they have been deducting +- R1600 and it's really costing me a lot of money