It's BT. I don't think there is huge difference whether you talk or it is just on, but I did not check. I did check between BT on and BT off in idle and that made big difference. I have 3200 mAh on old 6700 this thing can last forever. With mere 1500-2000 it's rough, BT is a luxury. Another offender is reception. If battery has been in use for few months capacity drops quite a bit.