Mtn trying to take me for a ride!

My iPhone 5 stopped charging, I took it back to them and after some struggle due to my girlfriend and I sharing a charger they took the phone in to be repaired. The replaced my iPhone with a new one and I was happy. 6 months later the new iPhone gave the same problem. I took it again, they gave me the same problems when I could not send it in with the charger as we only have 1 charger for both our phones, however they took it in. They then contacted me saying that there was a small crack on the phone above the camera (which I can't confirm as no check on the phone was done when I gave the phone to them), now they want me to pay R3000 to have the phone fixed! How can a crack that might have been caused by Mtn themselves affect the charging of the phone? I asked to speak to a manager or superior and a woman contacted me 2 weeks later just to inform me the exact info that I had already been given. I will not pay this money and I expect them to fix or replace the phone as it's got the same issue as before! I can be contacted on ***. Disgusting service as between me and my partner we pay over R1000 a month to Mtn!