SMSFlashWhat a one-time password (OTP) is, how SMS verification works behind the scenes, FlashCall and voice codes, and how OTP compares to authenticator apps.
OTP in one sentence
An OTP (one-time password) is a short code, usually 4 to 8 digits, that a service sends to your phone to prove you control that number. It works once and expires after a few minutes.
Why services use SMS verification
- To confirm the number really belongs to you during sign-up.
- To slow down automated fake accounts, since every account needs a working number.
- As a second login step (2FA) when you sign in from a new device.
- To confirm sensitive actions like changing a password or a payout.
How it works behind the scenes
When you enter your number, the service generates a random code, stores it for a few minutes, and asks an SMS gateway to deliver it. The gateway passes the message to the mobile network of your number, which delivers it to the SIM, or, for a virtual number, to the online inbox you see in your account. You type the code back and the service compares it.
SMS, FlashCall and voice
- SMS: the classic text message with the code.
- FlashCall: the service calls your number and hangs up; the code is the last digits of the caller ID.
- Voice: a robot calls and reads the code aloud.
SMSFlash shows which methods are available for each service and country when you order.
OTP by SMS vs authenticator apps
SMS codes are convenient but can be intercepted through SIM-swap attacks. For accounts that matter, add an authenticator app or a passkey as well. Use SMS mainly for sign-up and as a backup.
Receive an OTP online
Need a code without your personal SIM? Get a number for the app you need, such as WhatsApp or PayPal, and read the code in the browser. If it does not arrive, see 10 fixes.
Get a numberCreate free account
Frequently asked questions
How long is an OTP valid?
Usually 2 to 10 minutes, depending on the service. Always use the most recent code.
Is OTP the same as 2FA?
An OTP is one way to do two-factor authentication. 2FA can also use authenticator apps, passkeys or hardware keys.
Can I receive an OTP without a SIM card?
Yes. A virtual number receives the SMS online and shows the code in your account.