There are some third party apps that can accomplish this but only when using android dialer. I'm using your app on an android tablet so those third party apps fail. So I want as subject describes. A means of answering a call by just pushing the button on standard 3.5 mm ear buds that are plugged into the tablets audio jack.
Some of these earbuds have a volume rocker as well but I most at least have the one button that defaults to pausing/playing music.
If this can be done, what I think would be handy is:
Press button to answer call
Once answered, user can press to mute
Long-press to hang up.
But those last 2 would be nice to haves, just being able to answer without fiddling with the device would be great and I think a lot of users would like this feature and hopefully wouldn't be too difficult to implement.