MTN Wonderboom Junction lie when you take out contract with them.

I did a upgrade at MTN Wonderboom about 3 weeks ago. Chose a new phone and was told if there is something wrong wit the phone within 7 days they will replace it. Well the phone did not work properly and I took it back on day 3. They took my money for a early upgrade but here I am 3 weeks later and still don't have a new phone and they took the one back that was not working. I got hold of the Regional Manager Izak and he said he will came back to me. I'm still waiting. If I try to call him he don't awnser my calls. They do break the contract. I will not do bussness with MTN Wonderboom Junction again. What can I do to get them to help me. The worst thing is everyone is pulling up there shoulders. You can see they don't warry about there clients. I got 3 contracts with MTN and this is how they treat there customers. If there is any one that can help me to resolve this issue I'll appreciate it.