Non-smartphones had a proprietary OS (to handle the phone functionality, SMS, contacts, call history), but licensed Sun's J2ME and/or Qualcomm's BREW to run downloadable apps/games. I don't recall those manufacturers having smartphones before 2007.
> What company were you working for that gave you access to so many phones at once?
I worked for two mobile game publishers, Mforma (renamed to Hands-On Mobile) and Digital Chocolate. Prior to smartphones, mobile operators/carriers ran phone app stores like game consoles. They partnered with select game publishers to stock their app stores. Premiere publishers had to commit to deliver multiple games per month and support a minimum number of phones. For example, our delivery commitment to just one US carrier (Verizon) was two new games per month, supporting over 40 handsets. The handset list got updated every month. We received new phones weekly. By 2010 we had huge boxes filled with old discontinued phones and their proprietary chargers. Every carrier had unique handsets, each handset had unique bugs that thwarted you from putting a generic game build on it. It wasn't just differing screen sizes, they'd have different codes from the keyboards, vastly different memory and speed configurations. Because of these kinds of demands, during the 2000's mobile game publishers went through consolidation and acquisition of mobile game studios all over the world -- e.g. Mforma had studios in Manchester and Seoul; DChoc had studios in Finland and Barcelona, and set up porting shops in Bangalore and Mexicali. Supercell (Clash of Clans) was formed by former DChoc developers at the Finland studio.
> Did you ever work with Japanese phones models (NEC, Kyocera, etc.)?
Not really, only rare ones that were offered by US carriers, and only to port a game to it. But in 1999, I was in Japan at a reunion (I attended high school in Yokohama) and had a chance to play with a couple phones there. They were cool, very slim phones with all kinds of functionality.
> While you've said quite a bit about how easy the Palm was to use, which was the best one to work with? Or, at least, which one would have been a hacker's choice.
There were other different PDA platforms like Msft PocketPC and Sony Clié, but I had developed an app only for PalmOS. It was possible to use GCC[1], but I bought Metrowerks Codewarrior. I suppose nowadays that's like buying an IntelliJ IDEA license. (BTW, when I got into doing BREW development, the compiler cost $2500, and that was after a discount that Qualcomm got from ARM for a special limited-function edition of their compiler.)
Here's an example of how phone platforms were so behind on computing. PalmOS had its contacts/calendar/notes functionality on device but you could also run that on Windows/Mac in an environment called Palm Desktop. It wasn't obvious to phone manufacturers that they had to do anything similar. In 2005 I had a Nokia 6620 (Symbian Series 60), and accidentally deleted a contact. To restore it from a backup was convoluted. The last backup I had was a couple months old. It was a monolithic binary image that restored in one block to the phone, wiping out any information that was entered after the backup was made. So first I had backup the phone, restore the older backup, get the one phone number that I needed, then restore the backup I'd just made, and type in the phone number into the phone again. Later they developed something like Palm Desktop but it was years later.
[1] https://web.archive.org/web/20000819061955/http://www.palmos...