When a call comes in, you can answer it via a headset, speakerphone, or handset.
To answer a call using a handset
Lift the handset off-hook.
To answer a call using a speakerphone
Press the Speaker button.
To answer a call using a headset
Usage depends on how the headset is connected. Often, you'll press the button on the headset or press the headset button on the phone.