Mtn not admiting to their fault and blaming me

I got a contract from mtn for a Sony xperia z2. The cellphone was like new and it had a minor software fault. So i gave it in the mtn repair centre and when i got it back it was working fine. but i did not notice how they put the phone back in place. Everyone knows that an xperia is waterproof and is sealed from everywhere, 3-4 months had passed and i accidentally dropped water on my xperia and as the water dropped, the phone shut down. I contacted mtn that when i gave the phone in last time for repairs they had not sealed it back how it should be. So they took the xperia in for testing where i told them that it was not my fault that mtn messed it up and i deserve a new phone as i pay every month for it. Mtn took 2 weeks and did not call me even though they had checked, instead i had to call them. they said my ports were opened when the water dropped on it, which is a lie because i know it was closed and all the water went in from the back, and now mtn is refusing to give me a new xperia. For the past 2 weeks im getting the worse service from mtn and they are denial of what they did wrong and blaming me for it. IMEI no. *** Sony Xperia z2