[Verizon Samsung Galaxy S III] newuser134:
I just came to a feeling of "revelation" after reading through some of the AT&T threads about the AT&T Note working on T-Mobile, to the conclusion that if a phone is capable of gsm on ANY band, it MUST also be capable of wcdma on that same band (as long as the phone has wcdma capability on other bands and is not a pre-3G/pre-wcdma era phone, like old flip phones, and we all know our phone is capable of wcdma 2100 AND gsm 1900).

This is why: the difference between gsm and wcdma is frequency/spectrum bandwidth and software manipulation by the CPU, kind of like wave and mp3 files. All the signals go to the same tranceiver, so if the phone has hardware for gsm 1900, it already has the hardware and RF path for wcdma 1900. It also has the ability to take larger chunks of a gsm band and use software to decode a wider frequency portion and turn in into a higher bandwidth (speed) wcdma connection. It's the modem software that does this. This wouldn't work if we were concluding the same thing for interchangeability between gsm and LTE, because LTE uses MiMo (multiple-in Multiple-out connections), and that is a different method of reaching higher data rates, it is not just a different modulation scheme using a wider bandwidth, so gsm and LTE aren't interchangeable even on the same frequency, but gsm and wcdma are. That's how they were able to change 3G into HSPA+ "4G", and receive higher data rates just by changing software, that's why 3G gsm phones can usually also get HSPA+ speeds on AT&T, but they don't call it 4G, just H+. That's why T-Mobile is going to re-farm its EDGE network to run HSPA+ on 1900 MHz, otherwise it would use it for LTE. The antennae are already there, the difference is modifying how the band is used with the help of software. Compare it to putting a large picture on a network of multiple tv screens as one big picture, instead of on just one screen. With the right software and multiple screens (in our case CPU power), it can be done.

That is why the 3G gsm/wcdma capable Verizon iPad 3 (newest version) can also receive AT&T's HSPA+, because the modem software is there, or the same iPhone 4s, if unlocked, can run on Verizon's CDMA 850/1900 and AT&T's wcdma 850/1900 without any hardware differences.

The difference between CDMA, gsm and wcdma is just software to understand the modulation/demodulation and the width of the frequency band, notice how they all use the same frequencies (850/900/1800/1900)? LTE is totally different and requires different bands (like 700, 1700 - this is not the same as 1700 AWS part of the band that is used for t-Mobile's wcdma) and won't work with the others.

All that is needed is the right modem software to literally "patch" the i535 radio to understand wcdma modulation on 1900 MHz, the same way it does on 2100 MHz. Right now it can receive wcdma on 1900 MHz, but it means nothing to the phone, it needs the ability to "read" it, we already know it has the ability to decode wcdma signals.

The only factors that decide this, are provisioning (sim), RF hardware (we know it is there for 1900 MHz) AND, the right modem software. When we put in an AT&T sim, we've provided 2 of these requirement, the one missing is the modem file. If someone can write the correct modem / shuffle the right files onto the right partitions (it may not be just the modem partition as we have seen from the imei problems), I know this phone could do it. I hope this proves it to everyone else the way it just proved it to me. I think that's why the Note was able to run on t-mobile, it had the RF hardware, it just needed the software decoding. Call it a codec if you will, that's all that's missing.

Now, if anyone is able to figure out what is needed, that's a different question, but being able to flash AT&T or t-mobile modems is the very first step. Now we need to figure out if it will brick or not.