Store don't want to credit me

On the 11th Feb I bought a cellphone at truworths with my store card,within a month the phone was not working, it was freezing, I took it back to the store after 3weeks they called and say the phone it's been fixed,when I get there the phone was not fixed,they said they fixed the charging point ,I ask them why the charging cause the problem was not that it was freezing. And to my surprise everything was written down what was the problem, so a told them I don't need a phone anymore cause it seems this phone it's going to give me a problem, now they comes a problem the store doesn't want to credit me they say they don't credit phones.now am paying for something that I don't have and I only use the phone for 2 to 3 weeks and it's stop work.Please help me how to deal with the situation.