its very simple
IC MT8870 gives 4 bit digital output corresponding to key press form cell phone.
so first wait for any change in input on port A of ATmega16. when input is changed
compare it with the code like 02, 04, 06 and 08.
based on match found give output on port D to rotate motors.
like if received code is 02 then rotate both motor in CW direction to move forward
like wise for all four keys you can do