Bad Service from PEP/ Void Agreement and Refusal to Accept Fault

My little sister bought a cellphone - KICKA - at PEP stores exactly a week ago, she had no joy since she bought the phone and asked if we could return it and swap for a bigger and more efficient phone. We found out when we got to the store that this cannot be, cellphones cannot be returned. This was not explained to her on purchase, nor did she sign on the receipt that she accepts the terms of purchase but the Manager said she cannot help. We have proof of this receipt and acknowledgement from Olga from the store that she did not explain the terms of purchase to the customer as she expected the client would be happy with the chosen device. This is ******** and ofcos unacceptable as there is no agreement to the terms without knowledge and signature confirming such agreement to the terms of purchase.