Cellphone Contract

I took out a contract with Vodacom in 2016, withini 6 months the phone went dead. When I took the phone in, I was told it had a virus and needed software upgrade, which they did and I had to pay for. A week later the phone went dead and never turned on again, paid for a dead phone. Naive me decided to upgrade again in April this year only for the same thing to happen again in leass than a month. Received the phone on the 3rd of April, a week later it went dead for a day and turned on again. Last week did the same and took it in for repairs inly for it to be returned the same day with no explanation as to what is going on. What is wrong with your phones and is there a way to cancel this contract because it is not working for me? It's twice now in a row and it is different brands.