PEP Store Davenport Ignorance Regarding an Adult

My mother was able to open an account at PEP store at Davenport square, they said she was able to even take out an iPhone SE and pay per month. She took it and within a few days there were issues with it and she had to take it back, after sometime she went to get it back and they clearly hadn't fixed it. They tried giving it back to her and she suggested they tested it first before she signed anything, after testing it she found out that the phone wasn't working properly and it was even worse than before. The issue is that they're saying she's in arrears because of the phone. Now my question is, how can they say that when she didn't even take the phone back home with her and she didn't sign anything for that matter. Now that we are trying to reach out to them they aren't giving us any attention or giving us any proper assistant. There is just no progress whatsoever.