How do I disconnect Bluetooth headset while in call
Depending on your phone (please update your question with your specific model), the call screen does have an option to turn off Bluetooth. That will turn off Bluetooth Audio Routing for just that current call (and conference calls made while that one is active/on hold).

I've only used CyanogenMod; CM6 &amp; CM7 both have this option on the call screen. If your phone's UI has been customized by the vendor, it may have been removed.