@dr_frost_dk

I am not sure the answer to your issue, but I can provide one datapoint. When my n900 is connected to the Sony bluetooth headunit in my car it switches automatically between playing audio with A2DP/AVRCP to HSP/HFP at the start and end of a call. I have not modified n900 stock settings in this regard, so perhaps keeping a BT headset around may be a crude solution.