Faulty (NEW) cellphone and bad Service

Wow when you upgrade your cellphone -you expect to get a New (WORKING) cellphone. Well not in this case. We Received a NEW Sony Experia M4 Aqua Dual Sim On Monday Switched it on - it was working - Switched the phone off to put it on Charge as they say one should do with a new phone- Plugged it in - no light Cellphone did'nt switch on but the whole cellphone top half became burning hot. Called Cell C Retension Department to asked them what we should do seeing as the phone wasnt working. They said they would come and Collect the phone to See whats wrong. On Tuesday the came to collect the phone . On Thursday we phoned to find out whats the next step. They said that there was absolutely Nothing wrong with the phone- they switched it on - they charged it everything was working fine. Well On Friday afternoon they delivered the same phone back to us - Battery full. Going through the settings personalised it etc. It wasn't used much - 5 hours later - SURPRISE-SURPRISE phone went off -didn't want to switch on- put it on charge to see if something would happen - NOTHING. Now we are sitting with the exact same problem.