someone on other forum recommend below. 我唔敢試, 唔知得唔得
Access the service menu on your phone my dialing *#*#197328640#*#*
Choose:
[5] AUDIO -> [1] NB (VOICE CALL) -> [2] HEADSET -> [1] VOLUME -> [1] SRC Speech RX Volume -> [5] 5_lvl: xx
and change xx by pressing the menu button -> Key Input and then typing in a value between 0-100, where 100 is 100% = maximum. To save, you need to use the menu button -> Back. Press this several times till you exit the service menu.
Pressing the fixed back button won't save your new setting.