I think isolating the call stream to authorised apps makes a lot of sense. Using a accessibility feature to make accessibility work seems like the right solution to this problem. The initial hack (just injecting tones into the system) sounds like a bad solution to be honest.
For other advanced stuff you have to be the default dialer app which also makes a lot of sense. If you're going to be an accessible dialer, you may as well actually communicate to the system that that is what you are. With plenty of open source dialer apps to borrow from, I don't expect the burden to even be that high.
Interestingly, the post writes that the app communicates with the phone over ADB. I wonder if you could just inject keypad button presses using adb, so the app doesn't need to even be an accessibility service to get the tones across? Although I guess that only works if the dialer is in the foreground.
For other advanced stuff you have to be the default dialer app which also makes a lot of sense. If you're going to be an accessible dialer, you may as well actually communicate to the system that that is what you are. With plenty of open source dialer apps to borrow from, I don't expect the burden to even be that high.
Interestingly, the post writes that the app communicates with the phone over ADB. I wonder if you could just inject keypad button presses using adb, so the app doesn't need to even be an accessibility service to get the tones across? Although I guess that only works if the dialer is in the foreground.