notedigo:
There are four different Nokia handsets that are called Nokia 5230. They are:
- RM-588: GSM 850/900/1800/1900 + UMTS Band VIII (900) / I (2100) EURO/MEA/APAC markets
- RM-593: GSM 850/900/1800/1900 + UMTS Band V (850) / IV (AWS) for T-Mobile USA, Wind Mobile Canada and DAVE Wireless Canada
- RM-594: GSM 850/900/1800/1900 + UMTS Band V (850) / II (1900) for NA, LTA markets
- RM-629: GSM 850/900/1800/1900 + UMTS Band V (850) / I (2100) for Brazil
Indeed there are firmwares in the 20.x range for the RM-588, but if you have a T-mobile Nuron, your handset is actually RM-593, and since the hardware is different (modem uses different frequencies), you can't use RM-588 firmware on RM-593 platform.
Second thing to keep in mind: T-mobile customizes firmware of the handset pretty heavily, with t-zones, my faves, etc. If you revert to a non-t-mobile firmware, you will not have all of that on your phone. I use a generic phone with t-mobile SIM when I visit US and it works OK for me, but you might miss some of the t-mobile customizations.
You can type *#0000# on your Nokia handset to get information about the version of the firmware and the platform.
Now, Nokia just released 11.2.80 firmware for T-Mobile Nuron. You can start by upgrading to it, using Nokia Software Suite if you are running 10.x firmware.
One additional thing - you generally can not downgrade Nokia firmware (Well, it's possible under certain circumstances, but has high probability of bricking your handset).
If you still don't like what you see with the t-mobile 11.2.80 firmware, you can use Nemesis Software Suite, and change the product code of the handset to the product code of the Canadian Wind Mobile or Mobilicity (DAVE Wireless) phone, and flash firmware 12.8.95. Be sure to write down your Internet/MMS/Packet Data settings before you do that, since they will get overwritten by the settings in the firmware you install.
There is almost no difference between Mobilicity and Wind firmwares (I am running Mobilicity firmware on a Wind phone) - you will have a different start-up logo, some new/different bookmarks in the browser, with Wind firmware you will get 5 Wind branded wallpapers, and there will be provider specific network settings (MMS/WAP/Internet) that you will have to change back to t-mobile ones.
Otherwise the phone will be as generic as they get.
Note that once you upgrade to the Wind or Mobilicity firmware, you can't revert back to t-mobile firmware until t-mobile releases a firmware with higher or equal version to 12.8.95.
The procedure (and google and read up on this) is to use Nemesis to change the product ID to either 0597292 (Wind) or 0599497 (DAVE/Mobilicity), then use Nokia Software Suite to update your phone as usual. Record your t-mobile product code (Should be one of 0592823 0597507 or 0597970), in event you want to get back to the t-mobile fold once t-mobile updates the firwmare.
Good luck.
P.S. If you phone is unlocked, and you are sufficiently motivated, consider reading up on making your own custom Nokia firmwares. You want your phone unlocked, since that makes life easier when you deal with dead phone flashing.
One of the upsides of making your own firmware is that you can add ROMPatcher+ right into the firmware, making the phone hackable, etc, and fix some of the "features" of the current firmwares.
One of the things that bugs me in the current firmware, for example, is the SMS that gets sent to Nokia each time a new SIM card is installed in a phone. With custom firmware that "feature" could be disabled.