I'm not really sure what your trying to accomplish. Generally, with a Bluetooth headset, you would already have it enabled on your 8703 and the headset would be attatched to your ear, this would be the fastest way to answer the call. If your wanting to put the headset on after the call is in progress, then you should probably leave Bluetooth enabled all the time and then all you have to do is put the headset on and turn it on to switch over. If your wanting to stop using Bluetooth during a call, then just turn the headset off during the call to switch back to the handset.
I'm not sure if this makes any sense to you but I hope this helps.