What is this ‘Carrier Settings Update’ on my iPhone, iPad?

Re/code:

Oh God, what is this?That’s the first thought you might have when a notification appears on your iPhone or iPad prompting you to download a “Carrier Settings Update.”If you’re like me, you’ll shrug and tap the Update button (YOLO!). But others might be a little more wary, and want to know what the update is for and whether it’s safe to download before taking any action.Unfortunately, getting answers to these questions is much harder than it should be.

I had this happen while setting up a friend’s phone yesterday. Finding info on what it is was frustratingly difficult.