Man lernt ja nie aus.
Ich fürchte es liegt daran, dass der implementierte Bluetooth Stack im Radio das nicht kann / will.
Hast Du Dir das hier schon mal durchgelesen? Insbesondere das was da über HFP steht?
Allow HFP Prompts
Hands-free profile (HFP) allows navigation prompts to play through your car speakers when iPhone isn't selected as the audio source. When you pair your iPhone with Bluetooth, HFP turns on by default. If you don't see the HFP option, your car stereo might not support it. Check the user guide for your car to see if your stereo supports HFP.
If you don't see the HFP setting when your iPhone is connected to your stereo, check with the stereo manufacturer. The HFP feature depends on your car stereo and your iPhone or iPad needs to have iOS 7. 1 or later.