Cellphone not working

On the 1st of April 2015, I upgraded my phone from a Blackberry 9320 to 9720 at Mtn in Springs. I took the phone home and when I was home, I put my simcard and the phone didn't work. After three or four days I took the phone back to Mtn and told them that the phone is not working. They checked it putting other simcards and the phone didn't work properly, it was like it's a network problem. They said I must do a simswap because my simcard is old, and I comp****. They said I nust the phone home because the simcard takes sometime to be activated and I did that. Later at home, I tried again and the phone didn't work. I took the phone back and they said they are going to take it for assessment. They took it and after maybe two months, because there was a strike, Asanda told me that I must pay for the phone. How can I pay for a phone that didn't work? Please help.