MTN complaint

I bought a smart phone on the 27th November 2017 at Tygervalley mall in Edgars. The phone is from MTN. In January 23rd 2018, the phone suddenly went blank then I returned it to them so it can be fixed. On the 23rd February they called me to collect the phone, I discovered that it was doing the very same thing then I returned it back to them the following day (24 February 2018) until now. They don't know when it will be fixed as they are still waiting for the parts of the phone and the don't know how long it will take. They are refusing to give me a new phone but it's their phone that has a problem