I had a pretty ugly pink bluetooth headset that I'd never gotten around to using (mostly because it was pink). I was thinking of converting an old telephone into an 'internet phone' when it occurred to me that the phone headset was the only important part. As you dial an internet phone number with a computer, the keypad was basically for show.
I hacked the phone handset apart, and cut out the old hookup and some reinforcing plastic to make room for the new motherboard of the bluetooth headset.
As you can see in the picture above, the original speaker and microphone are still wired to the bluetooth module. I'll simply swap the old speaker and mic with the ones already inside the handset. There is a small button on the back of the bluetooth headset that is used to accept/end a call, and put the device into 'pairing' mode. I'll wire a new push-button to the outside of the phone.
I've also set it so the USB charging connector will also stick out the old phone cord jack. I'll hot-melt glue the board into place once all the wiring is done, then re-package the phone handset.
I also drilled a hole in the face of the microphone part of the handset, and inserted an LED into the hole. There is a surface mount LED on the bluetooth motherboard that can now be seen on the outside of the phone, using the LED as a light pipe. You can see it in the very fuzzy picture up next...
I can leave it in the charger forever, really. I've paired it with my iPhone and my computer (not at the same time...) and it sounds fine, if not a bit lower pitched (I imagine due to the inconsistencies of the new speaker/microphone). You only need to hit the little red button to accept a new incoming call, and it cradles in your shoulder just like old times. Thankfully, my shoulder does not also hit the red accept call button when in use.