I have been using unlocked phones from overseas (Europe) on my Softbank 3G contract for around 2 years now. Softbank's 900Mhz and 2100Mhz frequencies match up nicely with what most European spec phones are compatible with, and Softbank do not (at the time of writing) restrict their Opensoftbank APN by IMEI (the Andglobal APN does seem to be filtered however), making it a relatively simple procedure of slipping in the Softbank SIM card and setting up the APN correctly.
Last year I was using the fantastic and much underrated HTC One S, and this year in April I upgraded to the HTC One, which is hailed by many reviewers as the best smartphone made so far.
With the HTC One comes LTE support, but on my old 3G contract, there was no possibility to get LTE data (it may have been possible by setting the APN but charges would have been by the packet, so I never thought to try). So in the last month or so I started thinking how I could get LTE in Japan.
From a hardware point of view, the One supports LTE 800MHz(Band 20), 1800MHz(Band 3) and 2600MHz(Band 7). With LTE more so than with 3G, it is important if possible to check not only the stated frequencies but also the band names, as there are multiple bands in and around the same frequencies that may or may not be compatible.
In Japan, Docomo supposedly offers LTE at 800MHz. However, this is not Band 20 but Band 19, which operates on slightly different frequencies. Not only this, but Docomo's rollout of 800Mhz LTE is said to be extremely limited at this time.
Next up, Softbank. As with Docomo, Softbank's main LTE frequency is 2100MHz (Band 1), but they also have some (?) LTE coverage at 800Mhz; however, this is once again not Band 20, but Band 18 (the bands overlap to an extent, but the best outcome that could be expected is you get only one of uplink or downlink working, which everyone knows means no connection is practically possible).
KDDI is out, as always, since even if they had a compatible LTE band (they don't), their 3G network is CDMA, meaning it is incompatible with the GSM HTC One (or pretty much any unlocked phone you will import from abroad).
Finally, E-Mobile. E-Mobile offers LTE at 1700MHz, which is actually the 1800MHz band, AKA Band 3. Bingo! Just one problem: E-Mobile doesn't have any 3G frequencies that match the HTC One.
At this point, Softbank CEO Son-san comes to the rescue. Earlier this year, Softbank bought E-Mobile's parent company, E-Access, and gave Softbank LTE users access to E-Mobile's LTE frequencies as well as their own, marketing it as "Double LTE". This means, with the right Softbank contract, I can use Softbank's 3G network and E-Mobile's LTE network, where available.
So, how to get an LTE contract with Softbank? Easy! Get an iPhone! (Not a suggestion you are ever likely to hear me make again). Actually, Softbank now has quite a few models which support LTE, but I knew for sure that the iPhone would give me access to the "double LTE" I needed, plus the Japanese blogs I researched had all done the same thing with an iPhone SIM, not an Android 4G SIM, so there may be a chance that the Androids use a different, more restricted APN or frequency set.
Plan decided, I went to the nearest Softbank shop. The LTE compatible iPhone 5 should be cheap now, since the 5S and 5C go on sale in a week, I surmised; and indeed, the 16GB model was 1,680 a month over 2 years, with a special discount of, you guessed it, 1,680 a month, i.e. it was free!
I filled out all the paperwork, made my colour choice (black, of course), and listened to the lengthy explanations of tedious optional Softbank services that I will never want to use. I signed everything, then finally was asked for my ID to confirm that I was, indeed, myself. I produced my "Zairyu Card" (Foreign Resident's Card), and was shocked to be greeted with that Japanese sucking-air-through-the-teeth sound that always means whatever you just requested is less likely to happen than a month of Sundays.
"Sorry, we can't give you a 2-year contract. The payments run until 26 months from now, and you only have 25 months left on your Visa."
"But... I've been here 10 years, I work for a Japanese company, I'm paying with a Japanese credit card, and I have a Japanese wife" [i.e. I'm hardly likely to do a runner before the contract is up ]
"Yes, I see. Computer says no." (Or words to that effect. I was fuming at this point)
"So what does this mean? What can I do to get the Shiny Phone?"
"You'll need to pay the 40,000 yen price of the phone up front, and you don't get any discounts".
I didn't bother to try and reason with her any further, and just walked out without saying another word.
To cut a long story short, I then tried the Softbank counter in Yamada Denki / LABI in Oimachi. They again asked me for my Resident's Card. I offered my driving license instead, but they insisted; apparently since when I originally signed up to Softbank, I used the Resident's Card (actually Alien Card back then), I need to use that as ID forever more. So I proffered the card and waited nervously. "Thank you, here's your iPhone". Success! It seems that the visa rule is not really a rule, and different Softbank shops have their own interpretation of what is and isn't allowed. My hints to anyone trying to get a contract with Softbank would be: first, try and use any other form of ID possible if they will let you (one that doesn't show your visa status), and second, if you fail at one shop, try again at a different one.
Finally, I had my iPhone. Plus, Yamada threw in a FitBit Flex wristband, although I only later found out that this only works with Softbank's own "Healthcare" app, not with the normal FitBit app. Of course, the Healthcare app is only on iPhone, and requires a subscription (first 2 years are free though). So we'll see how much use that gets.
And... The main result: The HTC One works with Softbank LTE, and of course with 3G as before! I bought a nano-SIM to micro-SIM adapter in Akihabara, found the APN information for Softbank iPhone 5 LTE via Google, and that was it. 7 GB of 4G goodness a month to enjoy.
Softbank LTE Speed Test on the iPhone 5
Softbank LTE Speed Test on the HTC One - faster than the iPhone 5