I was thinking about them having a setting that will enable you to change the default device to use for calls. eg. headset or handset. if no headset available revert to handset.
When once would make a call on the phone, audio would be fed to the headset via bluetooth and then you would be able to hangup by pressing the hangup button on the headset or answer calls by pressing the pickup on the headset without having to touch the phone.
If one connected a headset in the middle of the call they could press the multi-function button and switch it to the headset.