A method to increase the call volume via the audio settings is applicable to those cases where the phone or tablet independently makes adjustments to the sound settings when connecting a headset or headphones.

It often happens that the phone does not return to the selected volume when removing the headphone plug. In this case, the user has to restore optimal value using the volume keys.
Instead of repeating this procedure each time you disconnect a headset or headphones, perform the following steps.

Go to settings on your tablet or phone, and select "Audio". In the opened settings menu, locate and select "auto ringer volume".
Disabling the option "auto ringer volume" you get all the standard volume settings that will not change when connecting/disconnecting the headphone or headset.
Adjust the ringer volume via the "Engineering menu". To enter on your phone or tablet in the so-called "menu Engineering", you must use the telephone keyboard to input the following character sequence *#*#3646633#*#* and press the call button.
In the opened menu, find "Audio" and navigate to "Normal Mode". In this section, select "Type", then select "Tone".
Under "Value" set optimal ringer volume.
In order to exit the engineering menu, you must click on the off button of the phone.