CUSTOMER COMPLAINT- FAULTY PHONE (XPERIA Z5)

On the 02/12/2015 I made my way to Vodacom Northgate to acquire a new handset . I chose the SONY XPERIA Z5 COMPACT BLACK without any hesitation. On the 4/12/2015 all my customers were complaining that they could hardly hear me because of an echo on their side that when we were conversing. On the 9/12/2015 I returned the phone to Vodacom Northgate and the technician confirmed that the echo existed and requested I book it in immediately. Nathi booked it in (SEQ No# ***) and I was told it would take 9 days before we receive an official diagnoses, however they confirmed I would receive a new handset without a hassle after the compulsory process because its just a factory fault. On the 16/12/2015 I made my way back to Vodacom because no one had gotten back to me with feedback- I requested to see Brandon Neil. I was told their was nothing wrong with the handset and it will be arriving back . On the 17/12/2015 I called to confirm if the phone arrived and he confirmed it had. He then tested the phone with issues and called me. Brandon confirmed that the echo and said however they cannot replace it. I refuse to take back a faulty handset that will not give me peace of mind. Please help