Is there a simple way to go from bluetooth to handset while driving?
While you are on a call? I can do this from my iPhone5 and I would guess if you went to the Tesla bluetooth menu and said disconnect it would go to the handset.
Of course, that would be illegal where I drive.
There's a virtual button on the iPhone's telephone function screen to switch back to the handset. That's the quickest way. You can also disconnect the phone on the Model S phone app, but this is a more awkward way to do it, and you will need to reconnect after terminating the call, or before the next time you originate a call from the car's app.
Android - I believe I've switched to and from the Model S audio system while in a call by using my GS3's Bluetooth/headset button in the phone status window.