UNWANTED PROBLEM IN MY NEW HANDSET

HI. I HAVE BOUGHT SONY XPERIA E1. THE PROBLEM OF HANGING STARTED IN A WEEK OF PURCHASING. ON CONSULTING THE CARE THEY TOLD US TO SHOW THERE PHONE IN SERVICE CENTER AND GET IT REPLACED. I HAVE SHOWN THEM BUT THEY TOLD THE HANDSET IS CHANGED IN 7 DAYS OF PURCHASING. THEY HAD SERVICED MY PHONE KEEPING IT IN CARE FOR 4 DAYS BUT STILL THE PROBLEM OF HANGING IS GOING ON.
PHONE MODEL – XPERIA E1
PHONE’S IMEI NO. – 352947065738920
PLEASE SEE TO THIS PROBLEM IMMEDIATELY. THANK YOU.