The R/T button toggles the "up" and "down" buttons. When in "R" mode, the buttons serve as seek up or seek down. When in "T" mode, the buttons scroll through entries in your BMW telephone.
The lower button serves three functions...I do not believe that this is specific to bluetooth.
The first function that it serves is that it makes or ends phone calls on your BMW phone.
The second function that it serves is for voice commands. You can do things like say:
* Dial Number
* Read phonebook
* Save name
* Route map (navigation)
* Scale 10 Miles (navigation)
I am not sure if voice command comes with bluetooth or with NAV -- I have both.
The third is that it will dial BMW Assist if you hold it in. This function does not work when you have voice commands enabled as that is also how you activate voice commands.