HTC Hero :: Bluetooth Radio Call?

Bought a HTC Hero and a DEH-Pioneer 7100BT bluetooth car radio. Once radio was installed my phone was detected and I have done all what was necessary to get the phone paired with the radio.

The phone is now paired with the radio and can stream music and adjusting the phonevolume also gets played through the car speakers wich all work perfectly.
The problem starts when there is an incoming or outgoing call and when I answer the call through the radio, the sounds dont go through the car speakers anymore. The opposing person can hear me but I cannot hear them because the carspeakers are silent. The only way to hear them was by attaching a headset to my phone only then could I hear them.

The main reason for buying this radio was that I would not need a headset at all. Is there a way to route sounds from incoming and outgoing calls also to the car's speakers?