Damaged Cell Phone

I have a cellphone contract with MTN for the past nine years.<br> I upgraded to a Sony Experia, Z1, Compact on 07/05/2014. Four month later the body cover started lifting, the phone was in good working order but I was concerned that dust would collect on the inside if left unattended. MTN came back and stated that there was damage to the phone and quoted me in the region of R 2 000 to have the phone fixed. I said no to the exorbitant quote and asked for the phone back. I received the phone back with the cover unexplicably closed, however the speaker now was so faint that it was a struggle to hear what someone was saying. Since I mainly used that phone for whatsap and other multimedia it was ok. At the beginning of the year the phone had to be connected to the charger to be able to switch it on. Since the beginning of March the phone refused to switch on. We sent it in and once again received a high quote. I have phones with both Cell C and Vodacom. We had other phones repaired before. I feel that the phone has been giving problems since we got it and for a relatively upmarket phone want it fixed for free, otherwise I am stopping the contract. Why pay for something I cant use?