The general fix is to access the service mode, and alter the default max volume for a headset call as
Enter this number *#*#197328640#*#*
This will open the service mode menu.
Select option  AUDIO
Select option  (VOICE CALL)
Select option  HEADSET
Select option  Volume
Select option  SRC Speech RX Volume
Select option  5_lvl : 87
Press the menu button (left side)
Choose Key Input
Enter a value between 0 and 100, with 100 the loudest setting.
Then press the menu button and select back this will save (don’t use the right side back key you normally use to navigate the phone or you will quit out of the settings).
Keep selecting the menu and back until you exit the service mode menu.
You will also note you can change volume settings for other things like Bluetooth, handsets, video call etc.
It may to beneficial to try this fix firstly as it would not involve you having to return your handset.