Mtn and iPhone headache

I upgraded my contract with mtn to an iPhone just over a year ago , I looked after the phone very well , no scratches or markings on the phone , last week it started to respond very slow and freezes , I decided to take it into Istore where they send me to mtn , I booked the phone in at mtn head office , they contacted me today telling me I need to pay R4000 for the repair , I do not understand how I need to pay so much for a phone with no physical damage and no one can tell me what’s wrong with the phone it’s on a 2 year contract witch means I need to pay for something I can not use also mtn makes your data expire if you don’t use it , Mtn headoffice doe t want to help me getting this resolved ,tried to contact Apple with no avail they don’t even bother to respond on hellopeter