cell c change the horrible experience you have given me please.

So I took my iphone to cell c for repairs, they brought it back in an even worse state and I sent it back as I wanted it the way I handed it in, they brought it back with the same problem it had when I sent it as they said they couldn't fix it (freezing), I took it to the istore to try and get help and I was advised that all stores that sell iphones know that the only people certified to fix iphones is istore and nobody else, I was told they couldn't help me as cell c has already tempered with the physical phone, now I am sitting with an iPhone that doesn't work, I am still paying the contract and I can't even take it to istore for repairs, I would like to know how I can get that fixed now as I cannot be paying for a phone that I cannot even get fixed.