I bought myself a BH-907 Reaction headset for my Lumia 920. Some remarks on usage, as the user guide is woefully insufficient:
When you pair the headset with the Nokia Lumia 920 you can assign a call action to the menu key.
You can Navigate to Settings -> Accessories -> Nokia Reaction BH-907 to see the actions that you can assign with the key.
Hopefully this has been of help.