When iPhone is connected to a Bluetooth device, you listen to calls through iPhone by doing one of the following:
Answer a call by tapping the iPhone screen.
During a call, tap Audio on iPhone. Choose iPhone to hear calls through iPhone or Speaker Phone to use the speakerphone.
Turn off Bluetooth. In Settings, choose General > Bluetooth and drag the switch to Off.
Turn off the Bluetooth device, or move out of range. You must be within about 30 feet of a Bluetooth device for it to be connected to iPhone.