FAULTY HANDSET AT ACKERMANS

I bought a cellphone from Ackermans Phalaborwa on 26/05/2018, the cashier explained that I have a 7 days to return the phone if it is faulty, this is also written at the back of the receipt. After 4 days I returned the phone to the store as it had a problem of switching itself on and off randomly, to my surprise the management of the store refuse to exchange the handset and insist that its working fine. They tell me that one of their staff members inserted her sim card and proved that its working fine, the phone has been in the store from the day I returned it until now. I have used the phone and I am 100% sure that the phone is faulty. I want Ackermans to live up to its exchange policy as indicated on the customer receipt. Ackermans can not force me to accept a faulty product.