Open menu. Next, open the folder "audio Settings". Sometimes this folder is nested in transitional Settings " phone" or just "Settings".
Moving buttons (up / down or left / right arrow, locate and select the line "Volume of the phone". Open it.
The "up" button raise the volume to the optimum level. Press "Yes" and exit the settings.
In some phones volume setting is controlled by the side buttons marked with arrows or with the keyboard during a call (right-left).