This is how it could be done.
But your Samsung SGH-P900 will not work mobile anymore ... be warned ! ;-)
Step 1: Disassemble the Device
As soon as the screws are removed the device could be opened. There are some plastic noses that could be opened easiely.
I like to connect to the keyboard to be able to control the device remotely. But the connector or the keyboard has got very tiny connectors. No chance to solder here without destoying the device.
I thought it would be best to solder directly on the button pads. Doing that the keyboard will get unusable and the device could never be put into it's initial state.
This is the point of no return!
Step 2: Hairy Device ...
All wires soldered, ready for connection.
Step 3: The Interface
To connect to each button I needed a latched port expander using 74LS259 and 74LS138 TTL ic's. To press the buttons I used some old CD4066 analog switches.
See the attached logic plan. I soldered the parts on an evaluation board with small wires. On the borad I put also a voltage regualtor LM78L05 and some capacitors. The power regulator is a standard application to get a regulated power for the ic's. I connected a old, unregulated AC-DC charger.
You can see the setup on the attached pictures.
Step 4: Remote Controlling
To control all this I needed to write a small program. It had been many years since I did i/o programming on MS-Windows pc's. I found out that i/o programming has changed on 32bit systems like Windows-XP.
It took some time to find an universal IO-Driver-Kit on the internet. Using this kit it is possible to do i/o programming using port commands in c.
The result is a small command line program that can "press" keys on the phone.
If you like, I can add the source code of the program and record a video of the usage ;-)