Pep Lakeside Mall uninterested in exchanging a faulty phone!

I bought a small phone for my daughter from Pep Lakeside Mall on Wednesday. Got home and realized a little later that the sim card wasn't reading. It started reading again randomly and stopped once again so i took it back on Thursday. Upon arrival, I explained what my issue is and the lady manning the cellphone desk proceeded to take the sim card out and put it back again. The phone read the sim card and upon rolling her eyes at me, the lady said "there's nothing wrong with this phone". I then reiterated to her and her manager, Thabo, that it reads the sim card but then suddenly stops reading. Thabo looked extensively irritated with having to stop chatting on his CAT smartphone and actually attend to me. I told them I'd gladly exchange the phone for a different one and pay the difference. Thabo, the manager, said they can't do an exchange as "there's nothing wrong with the phone". I then said I could wait and show them what I'm talking about and short of him practically telling me to go jump, he said I had to keep the phone despite my slip saying an exchange could be done within 7 working days. I'm now at home stuck with an excruciatingly slow phone that only reads my daughter's sim card when it feels like it! Horrible service.