Glossary: SMS Verification Terms
Plain definitions of the terms used across our guides — from OTP and E.164 to carrier lookups and delivery reports.
UpdatedOTP
A one-time password: a short code, usually 4–8 digits, that an app sends to prove you control a phone number or email. It works once and expires within minutes, which is why you should type it in as soon as it arrives on the inbox page.
SMS verification
A sign-up or login check where an app texts a code to a phone number and asks you to enter it. It confirms you can receive messages on that number, not who you are, which is why a temporary number can pass it.
Temporary phone number
A number you use for a short time to receive SMS, usually a verification code. Free temporary numbers are public and shared; rented ones are private for a set time. Neither is meant to replace a number you keep for years.
Public inbox
A web page that shows every SMS sent to a free number. Anyone who opens it can read the messages, which is why it suits low-stakes sign-ups only. On this site codes are hidden 10 minutes after they arrive.
VoIP number
A phone number issued by an internet phone service rather than a mobile carrier. It can receive SMS, but many apps detect VoIP ranges during a carrier lookup and refuse them for verification.
Non-VoIP number
A number issued by a mobile carrier to a real SIM card. Carrier lookups report it as mobile, so strict apps accept it far more often than VoIP or shared numbers. Private rentals here are non-VoIP.
Carrier lookup
A database query an app runs before sending a code. It returns the network that issued the number and the line type — mobile, landline or VoIP. Some lookups also return whether the number is active, often called an HLR lookup.
E.164 format
The international phone number format defined by ITU-T: a plus sign, the country code and the national number, up to 15 digits in total, with no spaces or leading zero. Most sign-up forms expect it.
Country code
The 1–3 digit prefix that identifies a country in an international number, such as +1 for the US and Canada or +44 for the UK. Forms with a country picker add it for you, so type only the national number.
A2P SMS
Application-to-person messaging: texts sent by a system, such as a verification code, rather than by a person. Carriers route and filter A2P traffic separately, which is one reason codes reach some countries faster than others.
DLR (delivery report)
A status a carrier returns to the sender when an SMS is delivered or fails. Apps use delivery reports to decide whether to retry or offer a voice call. A “sent” screen doesn’t mean the DLR was positive.
SMPP
Short Message Peer-to-Peer, the protocol most SMS gateways use to pass messages between apps and carriers. Verification codes usually travel over SMPP before they reach the number’s network.
Two-number rule
Our rule of thumb for missing codes: if two fresh numbers from the same country get no code within two minutes each, stop trying that country and switch to a backup one. More numbers from the same country rarely help, and each request raises the risk of a temporary lock.
2FA
Two-factor authentication: a second check at login, such as an authenticator app code or a passkey, on top of your password. Turning it on stops anyone who later gets a public number from logging in with an SMS code alone.
Put the terms to work: how to get a code on a free number · VoIP vs non-VoIP numbers · phone number formats.