If the audio from the iPod part of your iPhone is travelling out the dock connector whilst you have your iPhone paired to a Bluetooth headset (of any description) the call will cause the music to fade out and ring via the bluetooth system.
You will be able to use the Steering wheel controles (which control the stereo) to control the music playback and the Steering wheel controls (which control the bluetooth option) to answer calls.
Keep in mind that I don't know heaps about the MDI however I do know a bit about iPhones ... from the iPhones point of view - it doesn't know what its plugged into specifically really ... it just knows it has some audio out options via the dock - and is paired to a bluetooth "headset".
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