Faulty Sony phone not replaced

We upgraded on 1st Dec 2015 and only activated the phone 11 Dec. 13th we had problems with the phone and returned to Cellucity. They advised us it must be send in for repairs. The phone has been in for repairs four times now for the same problem and Vodacom still says there is nothing wrong with the phone. I've send a complaint to Vodacom but nothing happened. Cellucity tested the phone themselves and experienced the same problem. They've send it to repairs but repairs sent it back claiming there is nothing wrong. It is very frustrating cause we need the phone and get no service from Vodacom nor Sony. It is a Sony Aqua M4.