FNB - Ignored and told that I am lying

I received an sms from FNB telling me that I need to have R1400 available in my account at the end of the month to pay my credit card bill. So I made sure that that money was there. I knew that I owed more but figured that it was only due the following month. Instead of R1400, a much greater amount was processed. I had the full amount ready for this installment in another account so that I wouldn't spend it. I made sure that R1400 was ready as I was going by the information in the sms. When the large amount wasn't available, I was charged a lot of interest. Seeing this, I deposited the full amount into my account. It took some days to register. During this time I received another sms telling me that I owe R1400. I raised this with the credit card complaint centre. They asked for me to send the sms's that I received and then got back to me to tell that I'm lying. I feel that if there system is giving me incorrect information, that they are responsible for paying me back the interest that they charged me. I've followed this up and they are now silent. Not happy.