Bluetooth/Phone knocking out Nav

I was using the navigation system the other night when I received a call on my cell phone, which was routed through the car's speakers through the bluetooth system. While I was on the phone, I noticed that the nav system stopped functioning, it was not showing where to go or saying when to turn. When the call was over, I had to re-set the location on the fly. Is this supposed to happen? it seems like a lousy trade-off. If it is not supposed to happen, can someone please tell me if there is a way I can fix this error?