Vodacom gives poor service

I bought a new phone at the Vodacom store in Stellenbosch, but after 1 day it had audio problems. I took it back on day two and expected to get a new phone because of the 7 day warranty. They said that they must first check the phone to see it I am to blame and that will take 3-5 working days. After 5 days the manager informed me that I will get a new phone, but they are waiting for store. After another 4 day the manager told me that they recieved the new phone, but they are waiting for a reliece code from headoffice. After a total waiting of 15 days the manager told me that my phone is fix and that nothing is wrong with my phone. They offer me one month redaction on my contract if I take the phone that was faulty back. How is this fair? Why do they have a 7 day warranty, but don't want to apply it to me?