Accused of malicious damage

I took out a contract in January 2017. In May 2017 the phone's speaker started giving problems for eg. Couldn't hear people and then afterwards they couldn't hear me talking to them.This was not my only phone i had and even though i used this phone mostly for the data i got monthly, i also got minutes and couldn't hear anyone nor could they hear me.I took the phone in to Mtn at Canalwalk Cape Town to book it in and have it checked in and repaired.I didn't hear anything from Mtn for a month.I went back to them to find out what is happening with my phone. I was told the phone is not at the shop and they too havn't heard anything yet.I then phoned the call centre and the person on the other line told me he'll get back to me before the end of day.He never did.I then went back to Mtn a 2 weeks later and i was told the phone is in Johannesburg and still not back.After expressing my utter disgust with the service i was contacted by Mtn the following day, informing me they couldn't get hold if me and that my phone had damage that apparently was caused by me. This phone had no scratches on it nor did it fall whilst i had it. The phone was working when i tool it in except for the sound issues yet now it had damages to the value of R1200 and due to myself as they put it. Currently the phone is still at Mtn as i have not damaged it nor was i neglegent.This phone is still under 2 year warrantee as i am still paying the contract but Mtn is not able to fix it as Mtn expect me to pay for the damages.My question is how can Mtn just refuse to fix the phone and how are they able to tell me that i was negligent with it as this phone can't even be opened or the battery taken out as it's a sealed unit.The phone is a Htc desire.I want someone from Mtn that is in a position to make a decision to contact me as i'm paying for something i have no use of and is going to take this to the consumer protector if i get no joy.