Cell C wants me to pay for a factory fault device

Hallo Peter my name is Thabiso Makgetha and I am a Content Producer for SABC 1 . I am a terribly disappointed with Cell C. I have been using their service and have two topup contracts with them. In 2015 i took an iphone 5s with them, I only used the device for 8 months and it was over heating, I reported the device a number of times on their social media platform and in November of 2015 they gave me a replacement device and it was a Sony M4 Aqua. I used the device for 3 months and it started malfunctioning, the microphone and camera started malfunctioning. In January 2016 they booked the phone inn and after 7 days I was told that the manufacture is saying the device ca not be fixed, they gave me a new device which is a Samsung A5. Three days later I was called by the same cell c walk in center Buccleuch to come collect the damaged Sony M4 Aqua, I asked the consultant what am I suppose to do with the damaged device but she told me it's mine I need to come collect it, I refused but they called again the next day. I to went fetch it, I gave it to my child to play with it and she lost it. now I am getting calls that I need to pay for that phone and they even suspended my lines.