Mtn & core group ******

I've upgraded my contract from from Mtn in february 2018,after just 3 months of using the device it started giving problems,when i took it to the store,they sent it to there repair centre,repair centre then contacted me after 3 weeks with a quote of R4200.00 stating that the phone has been physically damaged,i did ask the consultant what was damaged because the phone still looks intact,no broken screens and no water in the device,but she could'nt tell me,i just feel its unfair to myself as a consumer and as a 8 year Mtn customer with 2 contracts to be taken for a ride like this,iv'e never ever taken any advantage of my warranty of my devices from them and for Mtn to just terminate my warranty like this is very shrewd and is a ******** move for such a big company,i'm really considering taking my contracts elsewhere