Voice OTP is a one-time password delivered by an automated phone call: a text-to-speech voice reads the verification code to the user, usually twice. It works on any phone, including landlines, and is used as an alternative or fallback when an SMS code does not arrive.
What is voice OTP?
Voice OTP (also called voice verification or call-based OTP) sends the same kind of short code as an SMS OTP, but through a voice call. The user's app or website triggers the call, the user answers, hears "Your verification code is 4 8 2 9 1 7", and types it in. It reaches people who cannot receive SMS: landline users, phones with SMS blocked, numbers on networks where A2P SMS is filtered, and users with visual impairments who prefer audio.
How does voice OTP work?
Your verification service generates a code, converts a message containing it to speech, and places an outbound call over a VoIP termination route to the user's number. When the call is answered, the audio plays; the user enters the code in your app, and your backend checks it.
- Trigger. The user asks for a code or the SMS times out, and your backend calls the voice OTP API.
- Text-to-speech. The provider builds the message in the user's language, reading digits slowly and repeating the code.
- Call setup. The call goes out over a voice route to the destination operator, showing a caller ID.
- Playback and verify. The user hears the code and types it; you verify it and expire it.
Because the call travels over normal voice routes, the quality factors of wholesale voice apply: answer rate, post-dial delay and whether the caller ID is delivered. See ASR, ACD and PDD.
Voice OTP vs SMS OTP
| SMS OTP | Voice OTP | |
|---|---|---|
| Reaches landlines | No | Yes |
| User effort | Low, with autofill on Android and iOS | Higher: answer, listen, type |
| Affected by SMS filtering and sender ID rules | Yes | No, but affected by call labelling and CLI rules |
| Answer problem | None | Users may ignore unknown numbers |
| Cost basis | Per message part | Per minute or per call, with billing increments |
| Fraud exposure | SMS pumping | Calls to premium or revenue-share numbers |
Most teams use SMS first and voice OTP as a fallback, as described in OTP fallback with voice and WhatsApp.
Why does caller ID matter for voice OTP?
People rarely answer calls from hidden or random numbers, so a voice OTP that arrives without a recognisable caller ID often goes unanswered. Use a route that delivers your CLI unchanged, ideally a number your users can recognise or a local number where that is allowed for your traffic. Hidden or altered numbers lower answer rates and can break national caller ID rules. See CLI vs non-CLI routes and local CLI explained. In the US, calls also carry STIR/SHAKEN attestation, covered in STIR/SHAKEN explained.
How much does voice OTP cost?
Voice OTP is billed like any outbound call: per minute at the rate for the destination and number type (mobile or fixed), using the route's billing increments. A typical voice OTP call lasts well under a minute, so increments such as 60/60 vs 1/1 change the real cost a lot. See billing increments. Mobile termination rates differ widely between countries, so compare the cost per verified user against SMS in each market.
Voice OTP fraud risks
Voice OTP opens a fraud path that SMS does not: attackers enter premium-rate or high-cost numbers in your signup form and trigger calls to them, then share the termination revenue. This is a form of International Revenue Share Fraud (IRSF). Defend with:
- Rate limits per number, IP, device and number prefix.
- Blocking destinations and number ranges where you have no users.
- Allowing voice OTP only after a failed SMS attempt, not as the first option for every country.
- Alerts on calls placed without a matching verification.
- Short maximum call duration.
Voice OTP API and routes
A voice OTP API usually takes the destination number, the code or message, language and caller ID, and returns a call ID and a status callback. Behind it sits a VoIP termination route with good answer rates and CLI delivery. VoIPMatcher sources both voice routes and A2P SMS routes from its carriers and manages them. Request a CLI voice route for OTP calls.
Frequently asked questions
What is a voice OTP call?
A voice OTP call is an automated phone call that reads a one-time verification code to the user with a text-to-speech voice, usually repeating it twice. The user types the code into the app or website to prove they control the phone number. It is used for landlines, for users who cannot receive SMS, and as a fallback.
Why did I receive a call reading out a code?
Usually because you, or someone typing your number, requested a verification code and chose a call, or because the SMS failed and the service switched to a voice call. If you did not request it, do not share the code with anyone, since it may be an attempt to take over your account.
Can voice OTP be used on landlines?
Yes, and that is one of its main advantages. SMS cannot reach most fixed lines, but a voice call can. Services that need to verify landline numbers, such as businesses or older users, often offer a call option. Fixed line termination rates differ from mobile rates, so check pricing for both number types.
Which languages does voice OTP support?
That depends on the provider's text-to-speech engine. Most providers support many major languages and some regional accents, and you choose the language per call, usually from the user's app settings or the country of the number. Use clear, slow digit reading and repeat the code, because mistakes are more likely by ear than in text.
Is voice OTP more secure than SMS OTP?
Not by much. Voice OTP avoids some SMS issues such as malware reading messages, but it is still tied to the phone number, so SIM swap and call forwarding can expose it, and users can be tricked into reading the code to a caller. For high-risk actions, app-based or passkey authentication is stronger.
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