Last verified: 2026-08-27 07:35 KST
A Korean phone number is not a communication product. It is the credential that Korean banking, government portals, delivery apps and payment services check before they will deal with you at all. Which is why the question “which SIM should I buy” is the wrong first question. The right one is: what documents do I have today, and what does that let me open?
The document requirements below are a KT 5G example, drawn from KT’s published subscriber terms. They are not a verified policy for SK Telecom, LG U+, budget carriers or every tourist SIM. Confirm your exact operator, plan, visa and application channel before paying.
Prepaid and postpaid are a document distinction, not a payment one
English guides frame 선불 (prepaid) and 후불 (postpaid) as pay-as-you-go versus monthly billing. That is true and almost irrelevant. What actually separates them is what identity document each will accept.
| Category | Prepaid (선불) | Postpaid (후불) |
|---|---|---|
| General foreign national | Passport or residence card | Residence card — or passport if you hold E-7, E-9, D-2 or D-4 |
| Overseas Korean (재외동포) | Passport or domestic residence report card | Domestic residence report card only |
| Diplomatic mission staff | Passport or mission ID | Passport or mission ID |
| US Forces Korea | Military ID, or driver’s licence plus military documents | Same |
Read the first row again. The universal advice that a residence card is mandatory for a postpaid line is not what the terms say. If your visa is E-7, E-9, D-2 or D-4, a passport can open a postpaid line — under conditions, which are the subject of the next section and which matter enormously.
One rule applies to every row: postpaid automatic payment must come from a domestic credit card or bank account in your own name. US Forces Korea personnel are the single stated exception, permitted to use a foreign credit card. Your foreign card will not set up autopay, and no amount of asking at the counter changes that.
The three-month cliff
The passport-based KT postpaid route has a time limit; it is not the same as buying a short-term tourist data SIM.
Passport-based postpaid activation is not available at any shop. Under KT’s linked 5G terms, it is permitted only through dealers that hold an agreement with the sending institution — a labour ministry or dispatch agency for E-7 and E-9, a study-abroad agency or university corporation for D-2 and D-4 — and only for applicants who hold a document guaranteeing long-term stay, such as a standard employment contract or a certificate of admission.
Then comes the part that ends badly for people who do not know it:
Within three months of a passport-based postpaid activation, the subscriber record must be re-registered under the residence card. If it is not, the line is suspended and terminated by the carrier. Checked 2026-08-27.
Terminated means the number is gone. And because the number is the identity credential behind your banking app, your payment apps and your government logins, losing it does not cost you a phone — it costs you the account recovery path for everything you set up with it.
So if you activated on a passport, put a calendar reminder at week ten. Not week twelve. Take your residence card back to the same dealer and have the record changed. Getting the card itself is covered in our residence card guide, and booking the appointment that produces it in our HiKorea appointment guide.
How many lines you are allowed
Foreign nationals are capped more tightly than Korean nationals, and the caps are written into the terms rather than left to the counter staff’s discretion.
- A foreign national subscribing to more than one line can be refused, unless a deposit is placed or the applicant meets the carrier’s preferred-customer criteria.
- Counted across all mobile carriers together, within any 180-day window: Korean nationals are capped at three lines, foreign nationals at two, and — this is the sharp edge — one line if the document presented was a passport. Lines cancelled during that window still count toward the total.
- An exception exists: applying in person at a carrier-designated direct store lifts the cap for individuals, foreign nationals included.
- Applications from foreign nationals through home-shopping or telemarketing channels are refused outright.
Checked 2026-08-27. The practical reading: if you are on a passport, you get one line, and burning it on a throwaway tourist number can block the line you actually need. Cancelling the first one does not free the slot within the window.
What happens when your visa runs out
Your mobile line is tied to your legal stay, and the terms say so directly. A foreign subscriber’s service can be suspended when the legal period of stay expires, or when it lapses after departure. It can also be suspended if the residence card number stops being valid.
There is a documented reprieve, and it is worth knowing before you need it. If your extension is under review, submitting the application receipt can defer the suspension. If the underlying situation is never resolved, the contract can still be terminated.
The lesson is procedural: the day you file a visa extension, the receipt is not just for immigration. Photograph it and keep it, because your carrier may ask for it. And if the suspension has already happened and your status has changed through departure or an expired stay, the terms require you to go through the same identity verification as a new activation to resume.
eSIM, and the trap in dual-SIM
KT’s terms define an eSIM as a subscriber identity module embedded in the handset as a chip, onto which a profile — the subscriber identification software carrying identifiers such as IMSI and ICC ID — is issued remotely. That definition matters more than it sounds, because it explains the two things newcomers get wrong.
First, a profile is issued to a device, not carried on a card you can move. Plan on the phone you will actually keep.
Second, if you run a dual-number product, the terms state that when the USIM and eSIM are separated so that the main line and the second number end up in different handsets, the line using that plan is suspended — and if the suspension continues for 180 days, the line is terminated. Splitting a dual-number setup across two phones is a documented way to lose a number.
A travel eSIM sold to tourists is a different animal from a carrier eSIM. It usually carries data only and no Korean number, which means it cannot receive an SMS verification code, which means it cannot open anything that requires Korean identity verification. That limitation is not a setting; there is nothing to switch on. The consequences for individual apps are mapped in our essential apps guide.
The prepaid balance you will lose
Most foreign residents follow the same path: prepaid on arrival, postpaid once the residence card lands. There is a cost in that transition that nobody mentions at the counter.
Under the number-portability rules in KT’s terms, when you port from prepaid to postpaid, any remaining balance on the prepaid number is not carried over and is not refunded. Whatever you topped up is gone.
The fix is trivial once you know: stop topping up in the weeks before your residence card is due, and run the balance down. There is one narrow consolation — if you port prepaid to postpaid and then withdraw from the new contract within the cooling-off window, the terms state the prepaid balance is restored. Porting the other direction, postpaid to prepaid, has no such protection.
One more small limit while you are in the terms: number changes are capped at two per line per three months, with exceptions for a confirmed lost handset or where the carrier accepts that a change is unavoidable because of stalking.
A sequence that does not waste your first month
- Before you fly, arrange a travel eSIM or airport pickup for data only. Treat it as a bridge, not a plan, and do not top it up heavily.
- Do not use your one passport-based line slot on a tourist product if you intend to stay. Within a 180-day window that slot may be all you get outside a direct store.
- If your employer or school is arranging a passport-based postpaid line, accept it — and set a week-ten reminder to re-register under your residence card.
- Open the bank account before the postpaid contract where you can, since autopay requires a domestic account or card in your own name. See our bank account guide.
- Test the verification you actually need — your bank app, a government portal — before you cancel the bridge line.
- Only then compare price. Budget carriers reselling the same three networks are the saving, and coverage is identical because the network is identical.
Transit fits in around this rather than depending on it; a transit card works with no phone contract at all, as covered in our T-money guide.
What we could not verify
Plan prices change constantly and are quoted differently at airport counters, direct stores and online. We are not publishing price tables we cannot check against a carrier page on the day you read this. Budget-carrier policies on foreign-national signup, English support and foreign payment cards vary by operator and are not covered by the terms cited here.
The detailed carrier rules above are from KT’s linked 5G terms. Other operators publish separate terms, and their document lists, payment methods and application channels need their own check. Ask the operator to identify the clause that applies to your product.
Verification note. On September 24, 2026 an AI-assisted check went back through the official sources linked in this guide (except KT’s terms-of-service PDF, which cannot be checked automatically) and confirmed each still resolves to the same official page. I publish this guide under my own name and am responsible for what it says. Rules and fees in Korea change often; if you spot something out of date, email hello@90dayskorea.com and it will be re-checked and corrected.
Sources
- KT — 5G Service Subscriber Terms (PDF): Article 6 refusal grounds, Appendix 2 required documents, foreign-subscriber suspension, number portability (Korean)
- Second source for the three-month rule — reporting on the three carriers’ 5G standard terms submitted to the Korea Communications and Media Commission, confirming the same re-registration and forced-termination provision applies beyond a single carrier (14 August 2026)
- KT — corporate site, terms and disclosures
- SK Telecom — official site (Korean)
- KT — official site (Korean)
Last verified: 2026-08-27 07:35 KST. Carrier terms are revised several times a year. Where this page states a rule, it is drawn from the terms document linked above as published on that date; check the current version before a decision that depends on it.