When using the headset the volume of your calls is controlled by Settings/Sound/Volume/Media.
This needs to be set before you start the call. You cannot change it once the call is in progress.
The 'In'call volume' accessed by the rocking swich on the left of the phone will attenuate the volume but will only allow you to send and receive audio to the maximum set on the 'Media' volume.
This is not an ideal solution because:
If you are listening to music on the music player set to for example 10 [the maximum is 15
] and you recive a call the volume of the call will be at 10 and you cannot change it.
My guess is that the Samsung guy who designed the dialer app was in a different room from the guy who designed the Settings/Sound/Volume dialog. and they were not on speaking terms...
The best solution would be for the dialer to use 'Media' volume instead of the useless 'In call volume'.
If I worked in the Samsung office I could go down the hall and tell the guy but instead here we are typing sentences on some obscure forum that i doubt the Samsung guys even read.
So good luck and if you find a better solution share it please.