The OKI-900 is the phone most favored by cellular experimenters. A company called Network Wizards has produced a hardware/software package for the 900 and 1150 called The Cellular Telephone Experimenters Kit (CTEK). OkiPad is a Windows NT / Windows 95 utility for the Oki. The NAM is easily reprogrammed. Debug mode has several interesting functions and is accessable from the keypad without any special hardware. The 4711 modification for multiple ESNs requires changing the firmware. Techies may find the map of the prom useful. The OKI 900 uses an 8051 microcontroller. If you want to use the data port on the bottom of the phone, you'll need to know the pin-outs.
Famous OKI-900's in history.
Read the OKI 900 FAQ
Here is a list of the different hacked PROMs that have been made. never tried them myself though.
The OKI PROM Hacking guide by oki dokie. Probably the best cellular file ever written. Complete instructions for 4715 mod, detailed memory map.
If your news-server actually will let you, check out alt.cellular.oki.900