You can keep your phone from home, buy one outright, or pay for one monthly through a carrier. Each choice has a catch.
Using your phone from home
It must be unlocked (not tied to one carrier at home). Ask your home carrier before you leave.
It must support US network bands. Most recent phones from big brands do. Many US carriers have an online compatibility checker where you type your phone's IMEI (dial *#06# to see it).
If it supports eSIM, you can add a US plan without a physical SIM card.
An unlocked phone works with any US carrier and abroad. You pay the full price once.
You can then use a cheap prepaid plan without a credit check.
US iPhone models since the iPhone 14 have no SIM card tray. They use eSIM only. This works in the US, but check that your home country carrier supports eSIM if you will use it there.
Refurbished (repaired and tested) phones from the maker, big stores or carriers cost less and usually come with a warranty.
Financing a phone through a carrier
Carriers advertise "free" or "$0 down" phones.
You usually pay the phone in monthly installments over 24 to 36 months, on a postpaid plan.
This needs a credit check. Without US credit or an SSN, you may need a large deposit or may not qualify.
The "free" part often comes as monthly bill credits. If you leave the carrier early, you lose the credits and must pay the rest of the phone.
The phone stays locked to that carrier for a period, usually until it is paid off. Carriers set their own unlock policies; ask before you buy.
Buy from a store with a return policy, or meet a seller in a safe public place.
Check the IMEI on a carrier's site before paying. A phone reported lost or stolen, or with unpaid installments, may be blacklisted and will not work.
Make sure the seller has removed their account (Apple ID or Google account). Otherwise the phone stays locked to them.
Carriers and stores sell monthly phone insurance. It is optional. Compare its cost and deductible with simply buying a good case and screen protector.
Signing a 36-month financing deal in week one, before knowing if you will stay with that carrier.
Buying a used phone that is blacklisted or still locked to someone's account.
Bringing a phone locked to a home carrier and discovering it at the airport.