This topic appeared on the Elmers forum some time late last year.
I'd like to point out that while this is an unknown sport in the States, we have been doing this for a while now.
Region 1 does not have the 33 cm ham band, so the only way we can use the surplus 900 MHz analog cell phones is to convert them for 23 cm. We are lucky to have a 450 MHz system in most countries, too, so there is no shortage of cell phones to be converted for 70 cm, either

Please have a look at our page:
http://oh3tr.ele.tut.fi/english/modifications.htmlIt is not so hard to write your own ham firmware for older phones which use processors like Zilog Z80, Inter 8031/8032/8051, Motorola 68HC11, Hitachi HD6303 etc.
You only have to find a fellow ham familiar of the processor and the schematic diagram of the phone. The rest is trivial.
73 de Benjamin OH3BK