My phone is a close relative of yours and I have to have a "bluetooth toggler" installed to accomplish what you're trying to do (if I understand your question). One (BlueMusic)
was just listed in the announcements yesterday. I didn't have very good luck with it though and there are free programs that do pretty much the same thing. It really is just about personal preference as to which program you prefer. What they all do is transfer the audio from your phone to your headset. We're talking mono BT right?
Check this page
out and see if BTAudio fits your needs. That's the one I use, and the price is right!
It's not as slick looking as BlueMusic but gets the job done and comes with three icons. One to turn the BTAudio program on (meaning to begin transferring your phone's audio to your BT headset), one to turn it off (meaning to pass the audio back to your phone's external speaker), and the third toggles back and forth between the two.
I've found the "on" and "off" to be more definitive. You can count on them doing what you want. Toggle always goes back and forth, so if you lose your transfer after a phone call you might have to toggle a couple times to get the sound back to your headset. If you just hit the on - it turns on! Another thing I prefer about this set of command icons is you can set up a speed dial and/or voice prompt to access them. You don't have to mess with any menus or button presses if you don't want to.
Devices: T-Mobile MDA (Cingular SIM), i-mate SP5m (sitting in a drawer at the moment while I attempt to migrate to PPC permanently), Cingular 3125 (unlocked and in use on T-Mobile), Motorola MPx220 (retired), Motorola MPx200 (retired)
Accessories: Plantronics Explorer 330 & 2G Patriot Micro-SD (use w/MDA), Kingston 2G mini-SD used in SP5m, Plantronics Discovery 640 & 2G San Disk micro-SD (use w/3125), Jabra A210 BT adapter + Motorola HS805 (retired)