I found my old OKI 900 analog mobile phone while looking for a ferrite rod in a junk box at my parents home. This unit was manufactured in 1991. I acquired it second-hand around 1993. It cost me about a months wages back then.
Googling this phone, it seems to have been a very hackable phone back in it's day. It's based on the 8051 MCU core and there are various ROMs available on the network to do all sorts of interesting things. However, right now I don't have any interest/time in making any mods to this. Indeed I couldn't even be bothered powering it up. So I'll take Dave Jones advice: "don't power it up, take it apart"... but only out of lazyiness :-)