[Galaxy S III I9305 (4G LTE + 2GB RAM) Q&A, Help & ] HarryTheHound:
OK, I did a crap load of research on this earlier in the week because I'm in the process of buying a few I930x phones for the family via eBay.

My understanding is that the hardware is identical but the ROMs in use on each version may affect network operability.

The issue gets clouded because data you find here and there on the interwebs tends to have conflicting information about the phone's specs.

Regardless of what specs you read on GSMArena or elsewhere, the ROMS for phones that Telstra and Optus provide are customised for their requirements.

The hardware may be identical to the that described elsewhere on the web, but the software isn't necessarily so. ROMs for the i9305 and the i9305T are also customised by other network providers around the planet, and thus may differ from the vanilla "International" versions.

I'll restrict my reply to LTE (4G) here, as all variants of the i9300 and i9300T appear to be compatible with 850/900/2100 3G UMTS spectrums used by the three Australian networks.

Both Telstra and Optus currently operate 4G on the 1800 spectrum.

However, Optus operates additional 4G on the 2300 spectrum for its "4G Plus" customers - see Optus's notes at optus[dot]com[dot]au/network/mobile/4g

Telstra does not operate on the 2300 spectrum and has no requirement that its handsets operate there. Hence, I would be surprised if Telstra feels any desperate need to incorporate access to the 2300 spectrum in its ROM.

On the other hand, Telstra has "... also been using some 2100MHz spectrum to provide extra capacity on [their] 3G 850MHz network using the latest generation of network radio equipment known as Software Defined Radio. As the number of LTE devices that are capable of LTE2100 increases, [they] can simply change these cells from 3G to 4G-LTE via software. This will help [Telstra] manage 4G capacity until [they] get...new spectrum. See Telstra's notes at exchange[dot]telstra[dot]com[dot]au/2013/05/07/mobile-spectrum-auction-result/.

So, both the i9305 and the i9305T will work on the 1800 spectrum, but the i9305T may or may not work on the 2300 spectrum and the i9305 may or may not work (for 4G) on the 2100 spectrum.

Theoretically, the i9305T is also optimised for the specific band within the 1800 spectrum that it needs to operate on. However, in practice, the handsets are tolerant enough such that this is not an issue.

Handsets provided by Telstra and Optus will definitely work on the relevant spectrums their respective networks utilize. i9305 devices provided by third party providers require investigation on a case by case basis.

However, the situation is about to become more complex.

At last year's ACMA auction both Telstra and Optus bought additional bandwidth in the 700 and 2500 spectrums for expansion of their LTE networks. See Telstra's info here exchange[dot]telstra[dot]com[dot]au/2013/05/07/mobile-spectrum-auction-result/ (again) and ACMA's info here www [dot]acma[dot]gov[dot]au/Industry/Spectrum/Digital-Dividend-700MHz-and-25Gz-Auction/Reallocation/digital-dividend-auction-results.

In the Telstra link provided above, Telstra also says "Firstly we’re seeing new devices that support the LTE900 MHz band (another low frequency band) so we’ll use this in areas where coverage and capacity makes sense. We’ll also deliver Australia’s first LTE-Advanced network and devices using LTE900+1800" and "We’ve also been using some 2100MHz spectrum to provide extra capacity on our 3G 850MHz network using the latest generation of network radio equipment known as Software Defined Radio. As the number of LTE devices that are capable of LTE2100 increases, we can simply change these cells from 3G to 4G-LTE via software. This will help us manage 4G capacity until we get the new spectrum."

So, in the not-too-distant-future, Telstra 4G devices will operate on bands in the 700, 900, 1800, 2500, and possibly 2100 spectrums, whereas Optus 4G devices will operate on bands in the 700, 1800, 2300 and 2500 spectrums,

A summary is here mobilenetworkguide[dot]com[dot]au/australian_mobile_phone_frequencies.html"]here[/URL].

The big question then is how are customers going to get firmware upgrades so that there devices can take advantage of the spectrum changes.

It would be reasonable to assume that Telstra and Optus will provide updates for newer devices they have provided. However, neither will provide firmware updates for devices acquired from elsewhere. Optus ROMs won't take full advantage of Telstra's network, and Telstra ROMs won't take full advantage of Optus's network. Other ROMS may or may not provide access to all the relevant bands,

So, Telstra may provide updates for i9305T units it has provided, and Optus may provide updates for i9300 devices it has provided. In order to receive these updates you will need to be running the appropriate Telstra/Optus Australian ROM on your device.

Devices acquired from 3rd party providers, whether they are i9305T, i9305, or anything else, will probably be left out in the cold.

You could, of course, root your device and load a suitable ROM that way. I could do it, you may be able to do it, but 99.5% of people out there would not be able to do it. And I really couldn't be bothered to do it on multiple handsets.

So for peace of mind, given these imminent changes I'm sticking with Telstra branded i9305Ts. It just seems to be easier that way.

PS to mods. I saw the message "To prevent spam on the XDA forums, ALL new users prevented from posting outside links in their messages, yada yada yada" However, I'm not spamming and the information I have linked to is on-topic, but it is obviously too voluminous to incorporate into the post. Hence I have taken the liberty of modifying the URLs of the links as a compromise, so that they are not hotlinks but people can still find the sources. I don't mean any disrespect. It's just that I went through this exact same process earlier this week, and found the source information to be really helpful