A Bluetooth headset doesn't have direct access to your device's contacts. It can answer/end a call, redial the last number called and work with voice commands if the phone's software has that feature. The Treo 700wx does have MS Voice Command, but that version doesn't support issuing voice commands over Bluetooth unfortunately.
That means you'd need to select the contact on the Treo then hit the dial command on the Treo.
-------------------- Lisa Gade Editor in Chief, MobileTechReview