Have a Parrot Ck3300, works fine with my Treo 650.
Have a Sony HBH-660 works fine with the Treo 650.

Problem is I want the Sony headset, to operate while I'm at work, then on leaving work before I get to the car I switch OFF the headset, and when I switch ON the ignition, I expected the Parrot car kit to take over, but no chance.
The Treo will ONLY work with whichever I paired last.

Does the Treo 650 only work with 1 paired headset/car kit, or am I doing something wrong.

The above arrangement was fine with my Sony P910i.