iphone 5s booked 2 times for same issue

I have a contract with vodacom it is 4 months old, my i phone 5s started rebooting and freezing i took it to an istore i was told to book it in with vodacom since a contract phone,well i booked it in with vodacom phone took 5 days and it was returned back when i checked same issue persist,i have now booked it again with vodacom i checked the status on repair center the phone is on dispatch,how can you fix the phone but it comes with the same issue?