MTN invoicing errors and poor service

I changed my contract in December. My contract is now for R469 for data, calls and SMS's. I did not require a new phone. I have, since then, been invoiced for a new phone, additional data and overcharged for the contract and the limits were not exceded. Invoices have been for R1111, R1112 and R833 instead of under R500. I have visited the branch twice and phoned the MTN help line 3 times and, on request, emailed the information to provided email addresses twice. I have been given reference numbers and promises and have even received an sms but the problem remains unresolved. I am owed over R1300 due to the over invoicing. I received R110 credit in my last invoice. After 3 months of trying to resolve the problem, this is all that has happened. I am still paying R201 for a phone I don't have and being charged R613 (all plus VAT) for a R469 contract. From my experiences at the shop and with the help line seems that MTN is *********** or just doesn't care. I also have to wonder if someone is using the phone I am paying for.