Confirm which overseas carriers are compatible with Docomo smartphones

Frequency Bands and carriers supported by Docomo Xperia A SO-04E in the Philippines

Using Docomo’s international roaming site, world wing, you can confirm which carriers supports your phone’s 3G frequency bands. If you have your phone unlocked, you can just get a SIM card directly from that particular carrier. Docomo's information is only in Japanese, but all you need to know is the name of the country in katakana and the model number of your phone.

As long as you’re not visiting North America, getting a fast data connection with an unlocked Japanese smartphone should be no problem. This is because Japan’s primary 3G band, UMTS Band 1 (aka 2100 MHz or “IMT”) is pretty much the world standard. If I take my Docomo Xperia A (SO-04E) to the  Philippines, India, the UK, Singapore, Australia, Thailand, China, South Africa, or Brazil, my phone is compatible with at least one 3G network. While LTE would be nice, there are few phones compatible across multiple carriers - 3G is more than sufficient.

The problem comes when visiting North America, where spectrum is allocated differently than the rest of the world. There are no compatible 3G networks in Canada, the US, or Mexico. You may see that “3G 850 Mhz” is listed as a 3G option for some carriers, like AT&T in the US, but without support for the “3G” option, you will be stuck with 2.5G (EDGE) or 2G (GPRS) speeds, which are unusable. None of the recent Docomo smartphones compatible with their World Wing international roaming will get a 3G signal in North America. 

Do not pay money for a “3G” connection in the US with an unlocked Docomo phone. You’ll only get 2G, which is too slow to be useful on a smartphone. Note that the Docomo iPhone 5S/5C is listed as incompatible with North American 3G (PDF)

According to Docomo's information, their iPhone 5 can't be used with 3G while roaming in the US or Canada.
(Docomo is geographically challenged and thinks Mexico is in South America.)

Just how unusable is 2.5G? Allow me to share a recent anecdote.

I keep a Nexus device with a radio optimised for US T-Mobile, which in the past had been the best carrier for brief trips to the US because they offer a $3/day 200 MB 3G package with unlimited voice and SMS. I’ve used this plan all over the US for the past few years. Once 200 MB is exceeded, you are throttled down to 2G speeds (supposedly) for the rest of the day.

On my most recent trip the to US, I discovered that T-Mobile has gotten more aggressive with network management. The throttles would roll over into the next day, such that I spent much of my time with the equivalent of and EDGE data connection. Mail with no attachments gets through just fine, which is about the only good thing I can say about the experience. Most services and webpages simply time out, including google maps. By the end, I had actually switched my plan to the $2/day 2G package because paying the extra buck was just not worth it.