Unffair contractual treatment

My phone got ****** in September 2015. At first I thought my son misplaced it and later the evening when I called my number it kept on stating wrong number. The next morning I called MTN and found out the phone was ****** and that quite a few international phone calls were made. The person that stole my phone made R5066 worth international calls. I opened up an incident number at the police station and submitted it to MTN. MTN refused to write off the R5000 as they said I reported it to late. I kept on explaining to them that I thought my son misplaced it but they were very unhelpful. In fact the people that assisted me were very cold, they did not seem to want to help at all and I kept in walking in a brick wall. I am stuck with this bill plus interest and no one to want to help. I have had this number since 2007 and to lose it now because of something that was not my fault just seems wrong. I am very disappointed in MTN and his employees. Please can some help me fight this battle?