What's the meaning of HKCU/Controlpanel/volume?

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Now i want to change volume during a call, and try to use the way of changing registry during a call.
Now i am confused by several items in registry.
there is several items in /HKCU/controlpanel/volume, include: screen, key, ringer, volume, mute
I had set the "ringer" and "volume" to different value, but it is not influence the volume during a call.
I am consufed by its meaning.
Who know that?
and who know how to mute the GSM call during a active call?
thanks

I believe the device will not pickup the settings unless you soft reset the device, open and close the volume settings in control panel or programatically send the device a notification that the value has changed as it does not poll these settings.

Then it is no way to change volume in a call?
Someone said TAPI does not work.

It must be there is a way to change volume.
Because I can change volume using the slide key at the left side.
Who know how to control that or intercept that action?
thanks