Tap 
at the bottom of the screen to display the Settings screen.
You can change ringtones, call sound quality and also turn on/off the keypad touch sound.
Note: In iOS 10.x.x, 11.x.x, or 12.x.x, "Integrated Call History" is displayed instead of "Integrated Calls."
Video Calls
Video calling function is available.
Click here for details of the video calling function.
About Call Sound Quality
If you select "Low," the sound quality deteriorates but interruptions will get less.
If you find calls interrupted, please try "Low" sound quality.