OBF -Cellphone returned within 24 hours

Good afternoon<br> <br> Kindly assist, i procured a cellphopne at vodacom southgate on friday afternoon 19 february 2016 following the approval from cellsure the insurance, i later discovered that the handset had a problem by frequently restarting itself every five to ten minutes, i also had to turn it on now and again, it also overheat at the back, i then decided to return the phone the following morning on saturday on saturday 20th feb 2016, To my surprise they refused to give me a new handset and sayn they will book it in for repairs and i will get it back after 7-21 days, I still dont understand why an out of box failure should be my problem as a client. please assist