The mechanism is pretty simple. On the OnStar units, the CDMA cell phone has to "auto-answer" the phone to open the voice path. Once answered, the inband modem has bi-directional access to the voice path. Commands are sent to the onboard processor to perform functions such as unlocking a door or fetching the current GPS LAT/LON. The command set can easily accomodate an command to mute audio output to the car interior and switch on a microphone. It is probably already implemented as a means of allowing an OnStar agent to communicate with an injured party who can not push the buttons. Air bag deployment initiates a call to the OnStar emergency number.