Upgrade to new Samsung S6 faulty but no exchange

I did an upgrade at Vodacom 4u. Received my phone. As soon as i got it. It switched off continously and black screened. Took back to Vodacom. They sent away to Samsung Vodacare Repairs. After 5 days came back rejected no fault found. Sent again. Again rejected. I have now lost my 7 day cool off period. Manager gave me my phone insisted all ok. Got home phone black screened and switched itself off every 15-20 minutes. Went back in...sent phone in for 3rd time. Still waiting. A phone exchange should have happened already. I am paying for a phone i dont have...disgusting!!!