MONTHLY PREMIUM INCREASES EVERY MONTH

i took two phones with cell c and the agreement was that i will pay R 225 for Sony experian and R200 for blackberry Z3 after few months paying i release that my monthly instalment was more on both phones, plus the Sony was booked for repairs more then 3 times, when i ask them they say i have subscribed to get it, which i did . i then ask them to cancel the service two months after my premium was more again . i called the call centre to ask and they give me attitude they did not help, for this months i decided to pay at net bank to avoid the extra fees which i didn't't get any proper explanation for . the payment was made on the 1 st of June 2015 two day after i received a sms from them saying my account is in arrears with R48 hence i paid R300 when i called to ask they say my monthly premium is now R348 failing to provide with a reason why my premium is high. I'm so tired of calling and going to their walk in shop and still don't get valid reason to why my phone is always blocked hence i pay every month