what determines whether BT button starts up voice dial?

[mlevin77]

I had a Pioneer stereo in my car and it was extremely convenient: hitting the phone button on the stereo immediately launched voice command through the radio's sound system, so that I could dial by voice using the phone's addressbook (or indeed issue other commands that voice command knew). My car was totaled in a crash and in the new vehicle there's a separate BT system which doesn't seem to do any of this - it pairs with my HTC Imagio and can call out through it's own (very limited) memory, but it won't activate voice dial on the phone. So I'm thinking of replacing it, but I need to know: what feature / aspect of the BT system is it that enables the stereo to connect to the phone and launch voice command through the stereo instead of only dialing? What signal is it sending to the phone that launches voicecommand rather than dial?

Thanks,

Mike