For those wondering about using the iPhone on T-Mobile USA's network and it's network compatibilty, I have the correct answer for you.
For quite a while using the iPhone (or any non tmobile device) on t-mobile's network would only run EDGE (2G) data which is quite mind numbingly slow compared to 3g/4g and 4g LTE data speeds. Most of you have wondered if this will ever change, well over the past few months it does look like that change will actually happen. People have been reporting that they have gotten 3G/4G data speeds on their unlocked iPhone (or other device) running on t-mobile's network. It turns out t-mobile usa is upgrading their spectrum frequecy to support 3G/4G on the 1900MHz band meaning that if your unlocked phone uses 1900MHz then you will be able to run 3g/4g on t-mobiles network (this applies to all AT&T usa devices and most international models).
T-Mobile USA has also announced a deal that has been made with apple to carry apple products (the iPhone). For most this sounds like great news, but it may not be exactly what is expected. T-Mobile will begin offering the iPhone (along with other devices) at full price with no contract but lower monthly cost (this will save you more money in the long run actually). You may also bring your unlocked device to t-mobile to take advantage of it's value and classic plans as well.
Now back onto the topic of iPhone and other unlocked device t-mobile support. When you use the iPhone on t-mobile usa and are in an area that supports 1900MHz you will see in the top left corner the t-mobile logo and the 3G data logo next to it. Most of you are thinking "awesome, I have 3G" while others using a 4S or iPhone 5 are wondering "why does it only say 3G? I thought this was a 4G/LTE phone", well let me explain why this is. T-Mobile does not carry the iPhone (currently) and apple does not have built in support for T-Mobile USA in the iPhone/iPad's carrier bundle root files. Because it's not an official carrier of the iPhone, T-Mobile USA is part of the unkown carrier bundle so all you will see regardless of what data your actually getting, all you will see displayed is the 3G data logo next to the carrier logo.
Now with that cleared up, lets move onto network compatibiltiy. If your using an iPhone 3g, 3gs or 4 you will only be able to use 3G data as these device are not capable of 4G data, but if your using an iPhone 4S or iPhone 5 you should be able to utilize both 3g and 4g data. But what about t-mobile's upcoming LTE data? will the iPhone 5 unlocked be able to use it? well the answer is yes but also no for certain models. If you have an iPhone 5 from AT&T USA and factory unlock it, you will be able to use t-mobile's LTE data once its available, but if your using an international iPhone 5 or Verizon USA's iPhone 5 then you will not. Why is that? well its simple, the reason why AT&T's iPhone 5 can use t-mobile's LTE network (when available) while others cannot is because of the LTE bands that the phone supports.
The AT&T USA iPhone 5 model number is A1428 and supports LTE bands 4 and 17 (t-mobile usa will use LTE band 4), verizon usa's iPhone 5 model number is A1429 and supports LTE bands 1, 3, 5, 13 and 25 and the international iPhone 5 model which is sold only outside the US has the model number A1429 (same as the CDMA based model) but only supports LTE bands 1, 3 and 5which are not compatible with t-mobile's one LTE band which is band 4. Apple recently "started" to sell unlocked iPhone 5's in the US which carries the model number A1428 (AT&T USA's model), but has actually sold them since launch (even though apple stated they not). All that was needed to do since 22-SEP-12 is to purchase an AT&T LOCKED iPhone 5 no contract and activate out of the box with anything but an AT&T sim card and there you go, you have a factory unlocked iPhone 5, but thats the past, now apple just admits they do sell it now.
But anyway, I hope that this post helps clear up any confusion about using an unlocked device on T-Mobile USA. The 1900MHz spectrum network upgrade applies to all devices that use 1900MHz, so its not just for the iPhone. I personally use an iPhone 5 originally locked to AT&T USA but factory unlocked it and now use it on T-Mobile USA, I have had absolutely 0 issues with it as far as calling, texting, MMS and data go. I do get 3g/4g data in some areas and when tested its an average of 8-10 mbps (fastest was 12.71mbps) so mostly 4G data and while in those areas i've noticed that MMS messages send almost instantly and the free wifi hotspot feature is actually usable (if you use an unlocked device on another carrier, if the phone supports a normally paid feature, you can utilize it for free without adding to your current plan such as wifi hotspot).
Provided below is the APN settings required for your iPhone to fully function on t-mobile usa (as of now iPhones and iPads running ios 6.0 are guaranteed to fully work without issue, ios 6.0.1/6.0.2 may cause sending mms messages to fail):
go to the settings app, general, cellular, cellular data network and enter these settings
MMS proxy: 220.127.116.11:8080
Max message size: 1048576
MMS UA prof URL: http://www.apple.com/mms/uaprof.rdf
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