Your phone has a headset socket. This is suitable for the connection of phone headsets that connect via a HIS headset cable.
If there are no currently connected call, a call answered or made by pressing an appearance button is connected either handsfree or through the headset according to the phone's audio path setting. See Default Handsfree Audio Path. The same applies to dialing a new call.
If there is already a connected call to which you are talking through the headset, answering another call by pressing the appearance button answers that call using the headset.