iPhone 4 pairing with Pioneer DEH-P8400BH

I recently bought the Pioneer DEH-P8400BH head unit for my vehicle and my iPhone 4 (OS 5.1) pairs fine, but when I get a call or dial out the caller says they can hear me just fine but nothing is coming out of my speakers. It will stream music through when not on a call but I have to switch back to the phone and use the regular speaker to hear anything during a call. Is there anyway around this issue or a special setting I need to put on my phone/radio to fix the problem. Any help would be greatly appreciated.

Thanks.