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Press the 
(speaker) button.
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Headset
If the phone is not in headset mode: Press the 
(headset) button.
If the phone is in headset mode: Either press the (headset) button on the phone, or the button on the headset itself.
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Answer Soft Key
Pressing the Answer soft key during an incoming call will answer the call using the handset, speakerphone, or headset depending on what mode the phone is in.
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