MTN DOUBLE BILLING - not acceptable and no refund yet

I was double debited in around Oct/Nov last year, since then I have been emailing MTN customer service for a refund on the double billing, it has taken them almost 5 months to now tell me that i was being billed in arrears and now I will be billed in advance, i had no communication from them regarding this billing change, with this being said, this is not acceptable, I have been debited every month without fail, and with the double debit , I should be in credit, where is the invoice for the double debit, i want my money back , this is not acceptable, I have been emailing and asking for someone in accounts to call me and a manager, when you call the manager is always busy and i was given an email address to email which i did, the manager did not even email me, this is shocking service, I am thinking of porting my number, I am due for upgrade, the only reason I did not upgrade was because I was not keep on any of the specials, maybe it is better this way, it does not seem that MTN is keen on my business anymore, I have been a client for more than 20 years