There are errors in the android 6.0 software
1.Wifi weak attracts
2.2g-3g-4g network very weak
3.Radio headphones low volume sound output
4.Speaking earpiece sounds low sound gives low
5.Sending errors to programs example WhatsApp sending image is closing error
6.When listening to radio music, the sound is suddenly reduced by 50%
Please request new immediate update of admin
Hope the below can help you.
"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."https://www.youtube.com/watch?v=nudrlosR2fg