I dont know of any way to prevent the volume from being adjusted by the user mid-call, but you can certainly request that the phone not remember the volume setting that the user left it at when the call ended with. You would do this by using the voice.volume.persist.* parameters in your configuration file, where * can be any one of 'handset', 'headset', or 'handsfree', depending on which volume you wish to lock down. Setting any one of those to "0" will cause the phone not to remember the last volume setting that the user left the handset, headset, or handsfree speaker at.
-- Nathan
