The most critical part of this hack is to intercept the keypad. Thankfully the phones use a conventional 8pin-keypad-style circuit, which makes this extremely easy :)
Pull out the keypad and start taking apart each layer. The only layer that is challenging is the plastic tan-colored enclosure. It is snap fit so you have to grab a flat-head screw driver and use brute force to detach it.
Just like the last part you can cut all the free wires. We don't need them.
** Remember to cut the grey ribbon cable or the keypad will not work as expected. **
Solder 8 wires to the pins on the keypad circuit.
Every phone has the same pinout on the keypad (numbered left to right).
1 – ROW 1
2 – ROW 2
3 – ROW 3
4 – COLUMN 0
5 – COLUMN 1
6 – COLUMN 2
7 – ROW 0
8 – NA
All the keypad code has been beautifully packaged into an Arduino Keypad Library. So all we need to do is connect pins to our Arduino. Specify the pins in the code. And the keypad should be working!
For our code we are using pins 30-36 of the Arduino Mega.
COLUMN 0 – pin30
COLUMN 1 – pin31
COLUMN 2 – pin32
ROW 0 – pin33
ROW 1 – pin34
ROW 2 – pin35
ROW 3 – pin36
NA – GND