New phone not working, hasn't been replaced

My mom upgraded at Mtn Grove on Friday 26 Feb. I took phone back Wednesday 2 March as it was freezing. Lady at Mtn tells me the phone should be fine and that if they take the phone in then and give me a new one the shop will have to pay if there is nothing wrong with the phone - trying to cover themselves instead of just taking the phone back. I took it back the Friday, they took the phone in telling me I'll receive the new phone when new stock comes in. I gave them the phone as well as the box with everything inside. My mom gets a call saying the little paper slide that fits over the box is need. Except that was thrown into recycling on the Thursday after being told on the Wednesday everything will be fine and that they never have issues with that phone (Hauwei p8). It's been nearly two weeks with no phone. They aren't getting back to us and if they do it's nothing helpful. Mtn gave us a faulty phone and it should be replaced! My mother is paying for this and has been a loyal customer for years. It's not our problem that the phone doesn't operate as it should. If they took it on the Wednesday and didn't tell me it would be fine this wouldn't have happened! Such bad service!!