There are too many use cases here. A2DP headphones can also be headsets and typically are. If you were listening to music loud on your A2DP, you would want the ringing to come through the headset, especially if you are 25 feet away from your device. You might also not want ringing on phone cause you don't want to disturb others, hence why you put on headset to start with. Probably need to check speck of bluetooth profiles to see what is the standard behavior (which I suspect is not specified).