Circuit/System Description
WARNING: This page is about a different car, the 2009 Cadillac STS. However, it is still accessible from the selected car via links, so may be relevant.
When an OnStar® keypress is made, a serial data message is sent to the audio system to mute all radio functions and output OnStar® originated audio. After the audio system is muted, the communication interface module (CIM) transmits signals to the audio system on the cellular telephone voice signal circuit, and returned to the module on the cellular telephone voice low reference circuit.