Their service is really bad

On the 24th of April 2018 at about 17:30,I bought a samsung z4 at Dunns which is in Diepsloot Mall.When I got to my place, I found out that I couldnt make calls with my phone,I couldnt download.On the 25th day of April 2018 around 17:00,I went back at Dunns to change the phone for a better one.They told me that they dont change phones yet the previous day when I was buying,they told me that should I experience any troubles with the phone within 7 days,I can bring it back either for exchange/refund.They then told me that I should leave my phone and come back when their manager is there.They gave me his contact details which are ********** .His name is Thulani.

Can you kindly assist get a refund/ exchange the phone