I got ****** by mtn

I have an existing contract with MTN. My tablet developed a software problem and the screen was also cracked. As someone who believes in genuine parts, I took my tablet to MTN at village mall in Kathu, Northern Cape for a genuine screen. I reported to them about both faults, scree n and software. Now, they took my tablet to their repairs center in Bloemfontein. The service people gave me a quote of R2100 and I told them to go ahead. After about a week they called me and said I should make a deposit into their account since my phone was fixed. My finances were not ok at that time until after 3 months. I paid my debt after 3months and they sent the phone back to MTN. NOW here is where the problem is, I went to fetch the phone and it looked good, I was happy. Until I switched my phone on to make calls. The phone was having the same software problem. It just switches off on its own in the middle of a call or freezes while using it or refuse to respond when trying to answer a call. I took the phone back to MTN the following morning and they took it back to their service supplier. Now they want me to pay again for a service I already paid for and not fixed. is that in any way fair? Help