no replacement

I UPGRADED MY CONTRACT ABOUT 7 MONTHS AGO. I TOOK THE SONY M2 AQUA. I HAD IT FOR ABOUT A MONTH WHEN IT STARTED GIVING ME THE FIRST PROBLEMS(FREEZING, CAN NOT UNLOCK, SPEAKER NOT WORKING) THEY SENT IT AWAY TO THE REPAIR CENTER. THE DID A SYSTEM UPDATE AND RETURNED SAYING NO FAULT FOUND. IT WAS SENT IN A TOTAL OF 6 TIMES NOW. EVERY TIME WITH THE SAME PROBLEM AND EVERY TIME WE SPOKE TO THE MANAGER AT THE SAME STORE AND IN STORE THEY WITNESS THE FAULTS ON THE PHONE. ONCE AGAIN IT IS NOW BEING SENT BACK SAYING NO FAULT WAS FOUND. A NEW DEVICE WAS REQUESTED THE SECOND TIME IT WENT IT BUT NO SUCH LUCK. ONCE AGAIN WE REQUESTED THE DEVICE TO BE REPLACED BUT THIS MATTER DOES NOT GET ESCALATED AND SORTED OUT, THIS IS TOTALLY UNACCEPTABLE. AS I AM PAYING MY MONTHLY INSTALLMENT BUT HAVE ENDLESS PROBLEMS.