Malfunction of a new phone

My mother bought two phone's at Truworths mall of the south one phone started to give us technical problem e.g heating up and it would freeze or just log off when busy with it mind you this was just less than three days ,so since it was under warranty for seven days for any functional faults we are allowed to bring it back for them to send the phone to repairs now three weeks later we go to do follow up to check if there's anything ..The phone is back in the store there's nothing wrong with it as they say why would we bring back the phone if it's not faulty and the one that was bought with it is working perfectly fine the own is even enjoying it. I spoke with to ladies from customer service they were not helpful I refuse to take a product that I'm not happy with I want a refund or new phone.my mother has been their customer for decades now and has never experienced such this is the first...The service there is great the stuff is friendly and welcoming my mother shops there a lot .we living in tough times now business are not doing well customer should be valued and treat with care .