Incorrect Invoicing

I have been Autopage customer for more than 10 years and still they do not care about their customers. Autopage Cellular has continued disappoint again. My contract expired with them in Jan 2015 and have no intention of upgrading with them. They have continued to bill me for device payment. Their invoice costing is inconsistent. After receiving an incorrect invoice, I logged a call (ref no. 603796- 04th March 2015) for my invoice to be credited and was promised that the April invoice will reflect the credit. My April invoice does not reflect the credit after again making a call on 07 April 2015 to follow up on my query.