To clarify: if your phone’s GPS detects you being in a country that we do not have certification for it will default to US/FCC settings. Your frequency/power settings have NOTHING to do with the phone number you choose — it’s all about your last detected GPS location. The last two posts above are both slightly incorrect, so just clarifying here how it works.
(And there is no way to manually set your frequencies; it’s all done automatically. So if you are in Malaysia today and defaulting to US/FCC settings, when you travel to Singapore, it’ll switch to Singapore settings because your GPS will know you’re there. That’s all!)