GS2 VOICE CALL Volume

Enter this number *#*#197328640#*#*#*
This will open the service mode menu.
Select option [5] AUDIO
Select option [1] (VOICE CALL)
Select option [1] HANDSET(聽筒) / [2] HEADSET(耳機)
Select option [1] Volume
Select option [1] SRC Speech RX Volume
Select option [5] 5_lvl : XX
Press the menu button (left side)
Choose Key Input
Enter a value between 0 and 100, with 100 the loudest setting.
Press ok
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.