Cell C summit bad

On the 2 July I brought a phone at cells C Greenstone. Phone mane is Summit Cell C after t day the phone become very slow I keep on getting message memory full and update need to be done I couldn't even open any application I took the phone back to celle c Greenstone none of the sale person couldn't update anything as the phone keep on rebooting. Have to leave the phone for repair now I am with out phone as cell c refuse to give me spare phone. One Agent was laughing telling all summit phone sales have the same problem . Why Cell C selling a broken phone to people