Incorrect Billing Amount

I have changed my subscription level with the company. They have billed me incorrectly for the new subscription. I've queried and they suspended my service. Thereafter continued billing me for 3 more months. When inquiring about this, they just say they have no record of my telephone calls. They have now handed me over for a service that they have not rendered. Furthermore I have requested invoices from this company for more than a year and have only now received it from the debt collector.