It would really depend on how you tap into the buttons, either transistors or mosfets doing the switching, or if you can just put 5 volts on a side of the button to trigger it
it would all depend on your phone, best thing to do would be to directly send commands to the module inside, but again that depends on the phone
look and see if anyone has done something similar with your phone, not neccesarily with an arduino but any microcontroller