I'm not sure if we can provide a VUI response before OOBE is completed (Assistant is set up) or the user has chosen the HFP handler app as the Assistant. Even if it's technically possible to detect the connection of a BT device that supports HFP and play some sort of audio message, there's some question whether we would want to do this. There's a certain amount of "creepy" factor involved if the user has already aborted from the Assistant setup but we start talking to them via Assistant voice to ask them to choose a handler app.