Controlling call volume How do I set the volume of a call (ideally while being on the call)?

I can find volume settings for media, alarm and ring but none of those appears to impress the voice volume on a call.

Calls are currently much too loud on my phone.

OnePlus X, Android 5.1.1
As far as I know, this could be changed simply by buttons on the side of the phone at the moment you are calling.