I've been looking into this as well, and the documentation available aside from the HFP spec itself is, well, not much - and not easy to find. Also, the HFP device I have seems to be a bit buggy, but I can't tell if it's me or the device. If you make any headway on this, please post it here (or PM if appropriate), and I'll do the same. Also, I've noticed a bug in which an rfcomm connection will hang and drop on the N900, and I'll be filing it shortly. If you come across that while working on this, please add any relevant info to the repor. Thanks