That's not going to happen unless you have the optional Plantronics headset lifter. The phone has no way of knowing you pressed the buttonj on the headset. The way that button is normally used, when you press it the headset turns on but also a mechanical device lifts the handset out of the cradle.
If you do have the lifter turn the HEADSET flag OFF, and plug the headset into the HANDSET jack (there's another jack on headset base that handset plugs into). The phone will then believe the headset is the handset, and when the handset lifts it will send audio...your headset base will send call to headset. If you don't press headset button, just pick up handset, the phone will still send audio but headset base will send to real handset.
