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ROM Software Downloads for Cingular 2125, Cingular 8125, T-Mobile USA SDA, T-Mobile USA MDA
The HTC Americas website is a wealth of helpful information. Among the other things you can find there are software downloads for a number of HTC devices. Currently, HTC have software for four of its most popular devices in the USA, including:

Cingular 2125 Software version 1.8.42.14
Cingular 8125 Software version 1.8.11.1 WWE
T-Mobile SDA Launch SW 1.9.19.16
T-Mobile MDA Launch SW 1.8.10.2 WWE

You may remember when the Cingular 2125 was first released, an update was quickly made available as early units somehow shipped with a non-standard baseline.

So, I'm not sure if the ROM Version 1.8.42.14 is newer than that first Update that HTC were offering. However, MobileTopSoft offer this: "It is supposed to fix some 3G SIM card issues, but before you rush off to flash your smartphone consider that the update doesn't have AKU2 and it doesn't appear to upgrade the OS at all (At the end You you are with the same ROM version). The only thing that does change is the Radio Version."

Source: HTC America, via MobileTopSoft