Enter service-mode, goto ‘Internals’ -> ‘Test Mode’.
In the appearing menu select
‘[8] Audio’ -> ‘BTH Config’ -> ‘[3]Call config’ -> ‘[1]BTH Rx Call Vol’.
Now enter ‘-’ via the keypad, ‘WRITE’ must appear in top. Enter the new dB-value in the textfield in bottom via the large keypad appearing, 3-digit format.
Change the value to +5.0 dB, should be enough for every BT headset/handsfree kit.
Enter it as ‘050′, then ‘OK’ twice.
Leave the submenu with ‘#’, then leave service-mode. The change should take effect immediately!.