Bluetooth car stereo-no mic. How to use phone's mic instead?

[gkool78]

I own a wp8.1 lumia 830 and I have just bought a Pioneer car digital media receiver with bluetooth. The Pioneer did not offer an Internal mic and an external one had to be installed which I did not do as it looked really ugly in my car. At that time I thought it would be somehow possible to use the phone's mic instead while using the Bluetooth connection only for the speakers but I did not find a way to do that. As this is obviously a software limitation does anyone know a workaround for this problem?