well as you know anytime a electronic device has to xmit it will draw alot more pwr from the battery then on standby due to the now the electronics it has to be in a sort of "active" mode then "passive".
Edit: example BlueAntV1 Specifications (200hrs standby/5hrs Talk "if you can even get that") talk about draining the battery during active mode this thing just sucks/drinks the battery pwr like water....
Things that might help...is keep backlight "off" during call....maybe slow down the clockspeed....there is a cab for that....just little things will add up and save you some battery pwr during call.