Broken cellphone

In March of this year, (2017) my father took out a 2 year contract with Vodacom and recieved 2 cellphones. One was given to my mother, the other to my sister. My father has since passed away and my mother has taken over the contract. Since March, the phone ( Samsung J5 Prime has broken 4 times. The phone just freezes and does not unfreeze. We took the phone back and Vodacom had to send it to Johannesburg all 4 times, where 5 internal parts had to be replaced the last time. The phone has been at the repair store almost more than it has been in my mother's use. It is clearly a flop phone but Vodacom/Samsung refuses to give her a new phone. Meanwhile, she has to keep paying the monthly contract bill and has lost a lot of data in the process. ( The phone had data loaded on everytime it broke) Vodacom's attitude is also vexing. They send us from one person to another, nobody able to help, while my mother pays for nothing because she can't use the phone. They give us numbers to call, using our own airtime, yet nothing has been resolved. We asked for a new phone but the request has been ignored. My mother is using an old, unreliable phone now because her "new" one is constantly in for repairs. This is most unsatisfactory. There is nothing wrong with our money, there should be nothing wrong with the phone. They also refuse to let her end the contract and threaten to give her an enormous bill of she decides to do so. The phone is currently broken AGAIN. That is why I am posting this on her behalf from my phone. We have lost a lot of money with this. Please help. Thanks