Incorrect Billing for the past 6 months

For the past 6 month, Telkom has been invoicing different amounts every month, and when I call them to ask them what is going on, after a long wait, I get told to speak to the store, and when I speak to the store, I get told that they have nothing to do with the invoices that are sent out. I have tried reversing these incorrect debit orders and paying for them in cash, as advised by the store manager, but then get told, and I get charged for reversing the debit orders. I have reached my limit in frustration, and have requested and I return their devices and we go our separate ways. I am so frustrated right now. PLEASE HELP !!!!