Faulty device

I have two contracts with MTN and the 2nd contract I took out with them is for a BB Z3 and I have been having problems with the device from since I received it. I received the package in December 2014 and in February 2015 the phone just froze and wouldn't do a thing. I called MTN and they picked up the phone and returned it two weeks later. keep in mind I am paying for it no one is doing a follow up on the device to tell me what is happening or when will I get it back. Now it is October and same phone has started with its **** again. Now it does not detect sim cards whatsoever. I feel like MTN is robbing me off my money selling me damaged goods. I do not want MTN to bring back the same phone because in less than a year it has broken twice. Its like buying a bran new car yet it gives you troubles starting and they will not give you another new car as your car is faulty from the factory itself. I want my phone and a new one as I am paying for it and asked for a new phone from the beginning not a used up phone and broken nogal. I have to make time to take phone to their shop as they will not come pick it up from work like the first time it broke. I want a new phone my MONEYS WORTH!!!!