Artist's commentary
Isn't GP02 also a Gundam? :-)
I used the NTT Hikari Portable I just happened to be carrying. You connect via wifi on the access device, then make calls via LINE or Skype. If I'd had a SIM-Free iPhone, I could have called with that, but even used SIM-Free ones are about ¥70,000.
For the SIM, they've got three telco counters all in a row at Taiwan's airport, so just pick whichever you like or some such. I bought one at the counter furthest in. I was just doing as the Ran-shama person said, so I don't remember the company's name. (^^;)