Before you start

An unlocked handset, or an eSIM-capable one
A US address for postpaid, but not for prepaid
A payment method — prepaid accepts foreign cards, postpaid usually does not

Step-by-step

1
Activate an eSIM before flying, or buy prepaid on arrival
A US eSIM activates before you land so you have a working number immediately for rides, deliveries and bank verification codes. No identity registration is required at any point.
OnlineWho: YouDay 1
2
Check Hawaiʻi coverage specifically, not just 'nationwide'
Carrier coverage maps flatten Hawaiʻi into a footnote. Oʻahu's urban core is well served by all the major networks, but the Koʻolau valleys, the North Shore and the interiors of the neighbour islands are patchier than the map suggests. If you will hike or travel between islands, check the specific carrier rather than trusting the word 'nationwide'.
OnlineWho: You
3
Start prepaid rather than fighting a credit check
Postpaid runs a credit check. With no US file you will be declined or asked for a deposit. Prepaid has no check and is usually cheaper for the same data allowance.
OnlineWho: You
4
Turn on Wireless Emergency Alerts and keep them on
Tsunami warnings, hurricane warnings and evacuation notices reach you through the federal alert system. Some categories can be silenced in settings and people do silence them after one loud test. In this state, leave them on — the January 2018 false ballistic missile alert made people distrust the channel, and the correct response is to know how it works rather than to switch it off.
Mobile appWho: YouWeek 1
5
Learn the monthly siren test and your evacuation zone
The statewide outdoor siren network is tested at 11:45 a.m. on the first working day of each month, so a siren at that time on that day is a test and a siren at any other time is not. Look up whether your home and workplace are inside a tsunami evacuation zone in the same sitting.
OnlineWho: YouWeek 1
6
Port rather than cancel if you switch later
Number portability is a legal right and works between carriers and MVNOs. Request the port from the new carrier — never cancel the old service first, or you lose the number.
OnlineWho: You

Documents you’ll need

Passport for postpaid; nothing at all for prepaid
US address for postpaid
A payment card — prepaid usually accepts foreign cards, postpaid rarely does

Things most newcomers don’t know

The 11:45 rule is the single most useful local fact about sirens.

Hawaiʻi tests its outdoor warning sirens at 11:45 a.m. on the first working day of every month. Knowing that turns a terrifying noise into a calendar entry — and, more importantly, tells you instantly that a siren on any other day is real. Newcomers who do not know it either panic monthly or learn to ignore sirens entirely, and the second is the dangerous outcome.

Source: Hawaiʻi Emergency Management Agency

Hawaiʻi is the state that sent a false missile alert, and that history is live.

On 13 January 2018 an erroneous ballistic missile alert went to every phone in the state and took 38 minutes to correct. It is why local trust in emergency messaging is complicated and why people cross-check official channels. Know which those channels are — the state emergency agency and the National Weather Service office in Honolulu — before you need them.

Source: community-reported

'Nationwide coverage' is written by people thinking about the mainland.

Every major carrier claims Hawaiʻi coverage and all of them serve urban Oʻahu well. The differences show up on the windward side, in the valleys and on the neighbour islands, which is exactly where you will be when you most want a working phone. Check the FCC's broadband and mobile maps for the addresses you will actually spend time at.

Source: FCC — National Broadband Map

The US has no SIM registration at all.

Most countries in this app require passport registration to activate a SIM. The US requires nothing whatsoever for prepaid — you can buy one in a supermarket and be online in minutes.

Source: FCC — consumer guides

Common mistakes to avoid

Silencing emergency alerts after one loud test, in a tsunami and hurricane state.
Assuming the monthly siren is an emergency, or assuming every siren is a test.
Applying for postpaid with no credit file and facing a large deposit.
Bringing a carrier-locked handset from abroad and discovering it on arrival.
Cancelling the old carrier before requesting the port, and losing the number.

Some of this may be out of date. Spotted something inaccurate? Help us keep it right for the next newcomer.