When a Bluetooth headset is paired and connected to your phone, you can conduct calls on the headset for hands-free conversation. During a call, you can opt to use the Bluetooth headset or just the phone.
When using the Bluetooth headset during a call, the Ongoing Call notification icon is blue in the Status Bar (the icon is green when using the phone).
Tap the Bluetooth button on the Call screen to switch the call from the phone to the Bluetooth device, or vice versa.