iphone 5 - not getting alert tone when using bluetooth

I am not getting any alert tone (voicemail, text, etc) when my iphone 5 is connected to my car's bluetooth. I do get the notification and the phone does vibrate, but there is no sound. Same is true for my wife who drives a different brand car. The strange thing is that if I am already on a phone call through the car's bluetooth and get a text while on the call, the alert tone does sound through the car's speakers. If anything, the tone should be silenced if a person is already on the phone as it causes a distraction during the conversation. But I would like to find out if there is any way to activate the alert sounds when the phone is connected through the car's bluetooth.