Pay As You Go issues

I have been a pre paid customer since my iPhone 5 contract expired and I do not want to upgrade my phone. I have had endless problems with the airtime and bundles I buy. Last night I bought R200 airtime from FNB, I then tried to purchase a R160 1GB bundle but keep getting told I have insufficient airtime. I keep getting warning messages that I am using data out of bundles but no matter the bundle amount that I try purchase I keep getting told that I do not have sufficient airtime. When I called MTN customer service to complain I get treated like an idiot and told my iPhone is using data and that's why my data is finished. Uh yes but that is not the problem! Wish MTN would sort themselves out and stop treating their customers like idiots. Sort your prepaid services out, your service s disgusting and I have endless problems with your prepaid!!!!