Phone number & SMS verification glossary
This glossary explains the terms you meet when you verify accounts by phone, choose a phone number or pay with stablecoins. Each definition is short and neutral, so you can quickly check what a word means before you pick a number or troubleshoot a code.
OTP (one-time password)
A code that is valid for a single login or action and usually expires after a few minutes. OTPs are commonly sent by SMS, voice call, email or generated in an authenticator app.
2FA (two-factor authentication)
A login method that asks for two different kinds of proof, typically a password plus a code sent by SMS or generated by an app. It makes an account harder to take over if the password leaks.
Verification code
A short code, usually 4 to 8 characters, sent to confirm that you control a phone number or email address. Apps send it at sign-up, at login from a new device or before sensitive changes.
SMS (Short Message Service)
The standard text messaging service on mobile networks. A single SMS carries up to 160 Latin characters, or fewer when other alphabets or emoji are used; longer texts are split and rejoined.
Voice verification
A way of delivering a verification code by an automated phone call that reads the code aloud. It is often offered as an alternative when an SMS does not arrive.
Flash call
A verification method where the service places a very short call to your number and checks the calling number, or part of it, instead of sending a text. The call is usually disconnected before you can answer.
SIM (Subscriber Identity Module)
A chip or card that identifies a subscriber to a mobile network and links a device to a phone number. Traditional SIMs are removable physical cards.
eSIM
An embedded SIM built into a device and set up by downloading a profile rather than inserting a card. It does the same job as a physical SIM.
Mobile number
A phone number from the range a country's numbering plan assigns to mobile services. Mobile numbers can receive SMS and calls and are the type most apps expect for verification.
Landline
A fixed-line telephone number tied to a location. Many landlines cannot receive SMS, so apps often do not accept them for text-message verification.
VoIP (Voice over Internet Protocol)
Technology that carries phone calls over the internet. VoIP numbers are issued for internet calling services and are often drawn from non-mobile ranges, which some apps restrict for verification.
Non-VoIP number
A number that is not classed as an internet telephony number, in practice a mobile number from a country's mobile range. Such numbers are generally treated like any subscriber's mobile number during verification.
Virtual number
A broad term for a phone number used without a dedicated physical SIM, usually through an app or website. It can refer to VoIP numbers, forwarded numbers or online mobile numbers, so the underlying number type varies.
Local number
A number that looks local to people in a particular country or area, using the local format and prefixes. Customers often find local numbers more familiar to call.
Toll-free number
A number that is free for the caller to dial, with the cost paid by the number's owner. Toll-free numbers usually cannot receive SMS in the usual way and are mostly used for customer service.
DID (Direct Inward Dialing)
A phone number that routes incoming calls directly to a specific line, extension or internet phone system. The term is common in business and VoIP telephony.
MSISDN
The technical name for the full international phone number of a mobile subscription, made up of the country code and the national number. It is what the network uses to route calls and SMS to a subscriber.
E.164
The international standard for phone number format published by the ITU. An E.164 number has at most 15 digits and is written with a plus sign and the country calling code, for example +44 followed by the national number.
Country calling code
The 1 to 3 digit prefix that identifies a country or region in international dialing, such as +1 or +44. It is placed after the plus sign in international format.
Trunk prefix
A digit, often 0, dialed before a national number within a country but dropped when dialing from abroad. For example, a number written with a leading 0 nationally loses that 0 after the country calling code.
International format
A phone number written with a plus sign, the country calling code and the national number without the trunk prefix. Apps usually expect numbers in this format or add the country code for you.
National format
A phone number written the way it is dialed inside its own country, often with a trunk prefix and local spacing. It omits the country calling code.
Short code
A short phone number, typically 4 to 6 digits, used by businesses to send and receive SMS at scale within one country. Verification messages are often sent from short codes.
Long code
A standard-length phone number used to send or receive SMS, as opposed to a short code. Personal mobile numbers are long codes.
Sender ID
The name or number shown as the sender of an SMS. It can be a phone number, a short code or an alphanumeric name such as a brand, depending on the country's rules.
A2P SMS (application-to-person)
Messages sent by software to a person, such as verification codes, alerts and notifications. Carriers often handle and filter A2P traffic differently from personal messages.
P2P SMS (person-to-person)
Ordinary text messages sent between two people. Network rules for P2P messaging generally assume conversational, low-volume use.
SMS gateway
A system that lets applications send and receive SMS through connections to mobile networks. Apps use SMS gateways to deliver verification codes.
SMS delivery report
A status message returned to the sender saying whether an SMS was delivered to the handset or failed. It is also called a delivery receipt.
Carrier
A company that operates a telecommunications network and provides mobile or fixed phone service. It is also called an operator or mobile network operator.
MVNO (mobile virtual network operator)
A mobile service provider that does not own its own radio network and instead uses another carrier's network under an agreement. MVNO subscribers receive normal mobile numbers.
Number porting
Moving an existing phone number from one provider to another while keeping the same number. Rules and timelines differ by country.
Number recycling
The practice of reassigning a phone number to a new subscriber after the previous user has given it up or let it expire. It is why numbers linked to important accounts should be kept active or removed from those accounts first.
Rate limit
A cap on how often an action can be repeated in a period, such as how many codes can be requested per hour. Hitting a rate limit usually means waiting before trying again.
Stablecoin
A cryptocurrency designed to keep a stable value, usually by tracking a currency such as the US dollar. Stablecoins are used for payments because their price does not swing like other crypto assets.
USDT (Tether)
A US dollar stablecoin issued by Tether. It exists on several blockchains, so the same token can be sent on different networks.
USDC (USD Coin)
A US dollar stablecoin issued by Circle. Like USDT, it is available on several blockchains.
TRC-20
The token standard on the Tron blockchain. USDT on Tron is a TRC-20 token and must be sent to a Tron address.
BEP-20
The token standard on BNB Smart Chain. Tokens sent as BEP-20 must go to an address on BNB Smart Chain, not another network.
Wallet address
A string of characters that identifies where cryptocurrency can be received on a particular blockchain. Addresses are network-specific, so funds must be sent on the network the address belongs to.
Blockchain confirmation
The point at which a transaction has been recorded in a block and accepted by the network. Payments are usually treated as complete after a set number of confirmations.
Frequently asked questions
What is the difference between an OTP and 2FA?
An OTP is a single-use code. 2FA is a login method that combines two kinds of proof, and an OTP is one common way to supply the second one.
What is the difference between VoIP and non-VoIP numbers?
VoIP numbers are issued for internet calling services and are often outside the mobile range. Non-VoIP numbers, in practice, are mobile numbers from a country's mobile range, which apps usually treat as standard.
How should I write a phone number for an app?
Use international format: a plus sign, the country calling code and the national number without the trunk prefix.
Why do some verification codes never arrive?
Common reasons include a wrong number format, rate limits after repeated requests, filtering of certain number types and temporary network delays. Waiting and trying again, or choosing voice verification, often helps.
Can I send USDT on any network?
No. The sender and receiver must use the same network, such as Tron (TRC-20) or BNB Smart Chain (BEP-20). Sending on the wrong network can delay or lose the payment.