Your phone has a headset socket located under the phone.  

Pressing the bst headset HEADSET key answers a call using the headset.

The key is lit when there is a call connected through the headset.

The key can also pressed to turn a call already answered using the handset or handsfree into a headset call.

If you are not currently connected to a call:

If the handset is off-hook, answering a call be pressing the alerting appearance button answers the call using the handset.

If the handset is on-hook, answering a call be pressing the alerting appearance button answers the call using the speaker or headset if one is connected, depending on which of the two modes was last used on a call.  

If there is already a connected call to which you are talking through the headset, answering another call by pressing the appearance button will answer that call using the headset.