Wrong billing & poor service

I have four topup accounts with fixed monthly instalments, R1020.00, with MTN. Each and every month i get a sms with my amount due and i pay it each month by EFT. On 08/09/2015 i received a sms stating that i owe R3800.00 and it is due by 25/09/2015. I went to my branch and they refered me to the 808 helpdesk. I have phoned in numerous times and they always say i must give 24hrs for feedback. I have sent emails to the customer care and complaints departments with no feedback. Today the lady at 808 said that i have to pay the amount. I am not going to pay it and i feel they are in breach of contract. If i dont pay they will suspend my lines and will give me a bad name, which i have never had. I am seriously considering changing to another service provider. This is poor service and i will spread the word about it.