will it work outside united states on other carriers?

Depends on the individual phone and the carrier. The software needs to be set up by the carrier to allow over seas use or it has to be "unlocked" Apple is starting to sell unlocked iPhones. They also need the capability to put in a SIM card. I would contact the carrier you will be switching the phone to over seas and ask their requirements. Unless your using the phone while on vacation then be careful of additional charges from the carrier. Data is usually the thing that hits the hardest but you can usually block data usage on your phone while traveling.