How can I speak privately with the Nokia Bluetooth Display Car Kit CK-15W?

It is possible to switch from handsfree to the mobile phone without interrupting the ongoing call. When connected via a Bluetooth connection, you can terminate the Bluetooth connection by pressing and holding the mute key during the call. This will transfer the call to your mobile phone, from where you can then transfer the call to a headset. Pressing and holding the mute key will re-connect the car kit with the mobile phone and transfer the call to the car kit.