Phone number and SIM glossary for people moving countries

Every field has words that its documents assume you already know. This page is the one place on the site where they are defined rather than used, in a paragraph each, so that a sentence in a carrier's terms stops being a guess.

The number itself

Mobile number portability (MNP)

Moving a phone number from one carrier to another while keeping the number. The network it runs on changes; the digits do not. Porting is the mechanism behind number parking, and it is the reason a number can be worth keeping when the service attached to it is not.

Porting window

The period during which a number can still be moved. It is a carrier and country matter rather than a universal rule, which is why the site reports it per carrier rather than as a general figure.

Number parking

Paying a service to hold a number rather than to use it. The number stays alive and continues to receive messages where the service supports that; what it stops doing is carrying calls on its own network. Google Voice is the common example in the United States, and its rules are on its card.

Number retention

An arrangement under which a carrier keeps an unused number alive rather than recycling it. Retention can be automatic, as with India's prepaid rule, where the regulator requires it while a small balance sits on the account; or a service the customer buys, as with China's 停机保号. The two systems are compared on all carriers.

Automatic Number Retention Scheme

The name Indian regulation gives to retention that happens without the customer asking: a prepaid connection with a balance above the stated threshold is not to be deactivated for non-usage, and the carrier may deduct a small amount to extend the silent period. It is defined in the Telecom Consumers Protection Regulations.

停机保号 (suspend-and-hold)

Chinese for a service a customer applies for in order to stop using a number while keeping the right to it. It is bought rather than granted, priced per month, and its terms are published province by province. The mechanics for the three Chinese networks are on keep your Chinese number active.

Safe custody

A carrier's productized version of suspend-and-hold, offered by name to postpaid customers rather than prepaid ones at the network that publishes it. See the Vi card for the one we could read.

Grace period

The window after one thing ends and before another begins. The term is used for at least two different clocks, which is why it causes confusion: a grace period after a plan's validity ends has to do with benefits, while the window after a number is deactivated has to do with whether the number survives. Reading which one a page means is most of the work.

Deactivation, disconnection, cancellation

Three words the cards on this site keep apart when the carrier's own page does. Deactivation is the earlier stage: the line stops working while the number is still associated with the account. Disconnection and cancellation are the later words, used where that page says the number is released. A card that does not publish the distinction does not borrow it from another carrier.

Freeze period

The time after a number is canceled during which it may not be given to another customer. On the Chinese pages, the 90-day sentence in the national standard sits in the fixed-line appendix. The mobile figure we could read is in two contracts, Beijing Mobile and Shanghai Telecom, and those contracts do not speak for China Unicom. The wording and the limit of it are on keep your Chinese number active.

Number recycling

The practice of issuing a canceled number to a new customer. It is what a freeze period exists to delay, and it is why a number you gave up may later belong to a stranger who receives your old verification messages.

Messages and codes

OTP (one-time PIN or one-time password)

A short code sent to prove that whoever is logging in controls the phone. Banks implement it as a requirement rather than a setting: one of the banks on this site describes its one-time authorization code as always on. The delivery failures are mapped on why OTP SMS fails abroad.

Verification code

The general term for the same thing outside banking: a short numeric or alphanumeric code that proves control of an email address or a phone number. Where the code is sent determines whether it will arrive, which is why the channel matters more than the code.

Short code

The five- or six-digit sender that bank codes and marketing messages arrive from, rather than a full phone number. Short codes are handled by carriers differently from ordinary messages and can be blocked at the carrier level; one of the banks on this site names that filtering as a failure its customers have to fix with their own carrier.

SMS, and why it is still used

The oldest text messaging standard, and the one that works on a phone without an app. Banks on this site still send codes over it. Whether a given code arrives is the subject of why OTP SMS fails abroad, not a property of the message format itself.

SIM swap

Moving a number to a new SIM, either by the account holder or by an attacker who convinces a carrier to do it. It is the attack that makes SMS-based verification weak, and it is also the reason a carrier will ask you for identification before reissuing a SIM.

Radio, roaming and SIM hardware

Roaming

Using your home carrier's service on a foreign network. The home carrier pays the foreign one, which is why roaming is billed and why its availability is a setting rather than a given.

International roaming pack (IR pack)

A prepaid bundle bought from the home carrier that enables or prices roaming for a period. Whether it is required in order to receive messages abroad differs by carrier, which is why the site reports it per carrier rather than as a rule; the Jio position is quoted on the India page.

Fair usage policy (FUP)

The clause in a plan that limits what unlimited means. A plan advertised as unlimited data will often specify a volume after which speeds are reduced, and it is the figure to read before comparing two prices.

Wi-Fi calling (VoWiFi)

Carrying calls and messages over a Wi-Fi connection rather than the mobile network. For someone abroad it matters because it can deliver traffic without a roaming agreement, and whether it can receive the messages you need is a carrier-by-carrier question.

eSIM

A SIM that is software rather than plastic: a profile installed on the phone instead of a card inserted into it. How many profiles a phone holds, and how that sits next to a physical SIM, is set out on SIM card, eSIM or US prepaid plan rather than as a single hardware rule.

Dual SIM, DSDS

A phone holding two line identities at once. Dual SIM means two SIM slots; DSDS means dual SIM dual standby, where both lines can receive while only one carries a call at a time. It is the configuration that lets a home number keep receiving codes while a local number supplies data.

MVNO (mobile virtual network operator)

A carrier that does not own the network it sells. It buys capacity from one of the network owners and resells it. The prepaid carriers named on getting a US phone plan without an SSN are of this kind.

Carrier unlock

Removing the software restriction that ties a phone to the network that sold it. An unlocked phone accepts another carrier's SIM or eSIM; a locked one does not, which is the first thing to check before relying on a local SIM after arrival.

Banking and access

Two-factor authentication (2FA), multi-factor authentication (MFA)

Requiring a second proof of identity beyond a password. The factors are conventionally grouped as something you know, something you have and something you are. SMS is one something-you-have factor the banks on this site still use; an authenticator app is another, and it does not depend on a text arriving.

Authenticator app, TOTP

An app that generates a time-based code locally, without a message being sent. Because nothing travels over the phone network, it avoids the delivery failures described on why OTP SMS fails abroad.

App approval, push notification

Approving a login inside the bank's own app rather than typing a code that arrived elsewhere. It depends on the app being installed and able to reach the internet, and on nothing about the phone number's carrier.

VOIP number

A number that belongs to an internet telephony service rather than to a SIM. Banks can see the difference before sending, and at least one of the banks on this site states that such numbers are not eligible to receive a code; the published positions are collected on which banks accept a VOIP number.

This site's own words

unverified

What this site writes when a fact could not be confirmed on an official page. It means we could not find it, and it does not mean the thing does not exist. Every table on the site uses the same word for the same situation, and the reasoning is set out on all carriers.

Last verified, next check

The two dates at the foot of every page. The first is the day someone last read the sources and confirmed the page against them; the second is when it is due to be read again. A rule without a date is a rumor, which is why no page on this site is published without both.

Rule card

This site's page type for a single carrier's rules on a single subject: a summary at the top, a timeline where the carrier publishes the days, the rule in the carrier's own words, and the sources. The cards live under keep the number you are leaving.