Using dock connector and bluetooth has issues

I use my iphone to play music through an ipod dock connector to the audio system in my car. However it appears that when I do this that I can not answer or take calls through my bluetooth headset. Each works just fine independent of each other, so when I am not connected to the car system, the bluetooth functions properly. When I am connected, the audio works flawlessly.

Is there any reason why these can not be used in conjunction? I do get a message about going to airplane mode when first connected to which I reply "no". Calls come in, music is played, but no bluetooth. I have to disconnect from the dock and answer.... just a bit dangerous 😉

Any help appreciated.