phone still under warranty

My device become faulty and was not due to any fault of my own. The device did not charge so naturally I thought it is a fault with the battery. I took the phone to an Iphone store<br> Manesh at the IRepair store in Cresta - Tel no ***. They are an auth. Iphone technician. The phone froze when a new battery was inserted and therefor I took it back to MTN as<br> it was around 6 months old and still under warranty. MTN doesn't want to repair to phone and want to charge over six thousand rand -this is not acceptable as the phone is under warranty and an Iphone technician worked on the phone. I need my phone for work and I do not have R6k to pay. When I took out the contract I was not informed that I should bring the phone (even for battery) back to MTN. It will appear that MTN wants to abuse me has a customer as I am a small consumer and they can force their hand.<br>