Via commenter Willy comes a post purporting to have LTE data working on an iPhone using Docomo's Xi network. The post is in Japanese, so I'll explain below in English. It seems he:
- Started a contract for Xi with a Docomo nanoSiM (aka nanoUIM) with an factory unlocked iPhone.
- Turned on data roaming (Not sure why - maybe because DCM is not in carrier.plist?).
- Set the APN to
- Used iTunes to restore the 6.0.1 firmware, downloadable from here:
If this works for a Docomo SIM, it should work for a b-mobile SIM. I don't have any idea why simply restoring the firmware would work. As always, do any of this sort of thing at your own risk!