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One of my favourite things about the iPhone is that it's fairly
painless to throw another SIM card into your iPhone, and have all of
your carriers settings just work (internet, MMS, voicemail, and
tethering if your carrier allows it). The way this magic works is that
Apple actually has the settings for all of the official iPhone carriers
preloaded into iOS. If you throw in a SIM from a supported carrier,
the settings are simply loaded and away you go. These are referred to
as carrier bundles (or, carrier settings according to Apple). ...