So the phone was SL45i and at the time firmware was accessible over USB and not signed. (Or maybe the loader bypassed it?) Either way, I started with IDA, got the basic ideas from a forum where people actually knew what they're doing and learned to understand disassembly and hand-patch the code as I got deeper into it. No special equipment, just 3 months of a summer break that... disappeared. I started from patching a jump here and there, then understanding how the menus worked and changing them a little bit, then writing some trivial "games".