refusal to replace faulty phone

I took a new contract from a vodacom outlet and received a Samsung j1. As per guarantee I returned it within 7days as an out of box failure as the phone kept freezing and the screen jumps and without even touching the screen the menu goes into different apps. I had to wait a week just to find out they refuse to replace the handset because according to them there is nothing wrong. I collected the handset and it still does the same. For a company that large to refuse to replace a faulty device within the legal allocated time is pathetic. And I am sure Samsung would not have a problem with replacement. They are quick to sign contracts that they don't stick to. But they refuse to cancel the contract when they don't comply with their own t and c's. I am tired of large companies who don't respect the customer who is loyal to them. I have more than the one contract yet I am not a valued customer by the looks of things.