If your Bluetooth headset is not detected during a mobile call, your device may have defaulted to its own audio input/output. However, it is possible to change this configuration while remaining connected to the call.
From the room where the call is occurring, press anywhere on the screen to bring up the details (Figure 1):
Figure 1 Details
2. Click on the Bluetooth icon (Figure 2):
Figure 2 Bluetooth Icon
3. Select the relevant Bluetooth option from the list (Figure 3):
Figure 3 Bluetooth Device
4. Return to the ongoing call and confirm the Bluetooth headset has now been set as the default input/output source