New upgraded phone faulty - no replacement

I upgraded 2 days ago, whereby the phone is faulty, battery life goes down very fast and phone gets warm en goes off and on by itself. Cell C indicated I must bring the phone in they must send it in, thereafter can I only get a new phone. I signed for a new phone which is faulty and can only receive a new phone after 7 - 14 days, which is unacceptable. I am paying for this phone and do not have a new working phone.