BleepingComputer.com: GO 730 Bluetooth

I have a Tomtom GO 730 and although voice instructions and playing mp3's work fine through the car stereo, it doesn't channel bluetooth phone calls. It's connected with an audio lead (FM transmitting facility just isn't good enough). Phone calls can be heard through the internal speaker, but it's not loud enough or clear enough for practical use. The settings on the unit itself indicates that it's correctly set up for line out (audio cable)for instruction and music, is it just not an option for phone as well?