Unresolved issues re data and hardware fees

In November 2014 our cell phone account with Nashua Mobile was taken over by MTN and we have had nothing but problems since with our billing. For November, December & January we were billed for data bundles but the data was not loaded. We were then charged out-of-bundle rates on un-capped data usage. Some credits have been passed, initially just for the data bundle charge but not the data usage. How can MTN charge for a service