Bad service

I recently bought a phone and my monthly installment incresed before I knew that my installment had increased I had paid the initial installment of 350 on the 14 of Dec and I received sms on the 27of Dec to inform me that my installment had increased to 471 which then I paid in 121 on the 28 of Dec to add to the 350 I had already paid now on the 10th I received sms that says 1170 due and payment must be made immediately when I called costomer care I'm told that my installment had increased to 820 which I did not know about and now they have added due to late payment.... Truworths is messing up my credit history its not my fault that I received sms that states that my new installment is 471 and my balance is 5000 I chose 12month paying method if I'm to pay 820 that is 6month option which I never choose truworths is unfair and they are messing up my credit record when I'm done making payments I'm permanently closing the acc never again