NEW PHONE SENT IN FOR REPAIRS

I upgraded my sons contract in Dec 2014. In February his phone stopped working (screen blank). It was handed in to MTN on 03.03.15 whom then sent it on to be repaired. More than a month later it was sent back with the response they will NOT fix it because some or other code or number is missing which has got to do with us how? MTN sent it back to repairs again and to date we and the MTN branch have had no response.If the phone cannot be repaired then a new one should to be issued to us as this is under contract and warranty?. Nothing out of the ordinary was done to the phone for it to be like this. My son is 18 and is a very careful and responsible person. He needs his phone. Can someone please just respond and let us know.