Yes you can (so long as your Bluetooth supports that). The functionality has been available on the iPhone since at least the 3GS (possibly the 3G, I don't remember).
I should add that I used to use this feature all the time, but it drains the battery on the Bluetooth device (or at least the one I was using) ridiculously fast. I find it easier to now just use a high quality corded headphones and mic combo. There's nothing quite so frustrating as to still have hours left in the work day and have no more access to music.