Bluetooth to Phone back to Bluetooth

[gyroslice]

I listen to my phone quite a bit over bluetooth (BlueTrip Aux - for iPhone and smartphones - Griffin Technology) while driving. However, when a call comes in I have to switch the option to change to the headset earpiece. When I hang up the phone automatically converts back to bluetooth. So I am looking for something (an app or whatever) that will have the phone always answer on the headset earpiece and upon hanging up go back to the bluetooth. Any suggestions or help would be appreciated. Thanks.