Hope this will help.
"To enter the engineers code go to phone and dial pad and enter the code.
You are now in engineer mode with the top tab telephony.
swipe across until you get to hardware testing.
you will see the drop down menu for receiver, hac, headset, speaker.
select either receiver or speaker
manually change UL gain to say 240.
scroll down the screen and hit the button Set.
go back using < key.
then select voice again just to check the setting for UL gain has changed
then press home and test by ringing someone or receiving a call."