Hit the settings button (gear icon) that is located on the bottom left hand side of your phones key pad.

Navigate to the right until you reach the Audio options (speaker icon).

Once at the audio section, scroll down until you reach the Audio Path option.

Ensure the desired option is selected when it comes to audio path that will be used when the speaker / headset button is pressed on your phones keypad.\
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Click Save once the appropriate option is selected.

You can now exit from the phones settings interface. Going forward, if headset was selected in the Audio Path section, dial tone and call audio will be presented in the headset by default if the headset/speaker button is pressed once. \
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If you press the button twice, dial tone will be heard from the phones speaker instead.

This guide provides step-by-step instructions on how to change the audio path on a Mitel 6900 Series IP Phone, allowing users to easily switch between using a headset or speaker. By following these steps, users can customize their phone's audio settings and easily toggle between the two options with the press of a button.

Changing Mitel 6900 Series IP Phone's Audio Path - Headset or Speaker