Voice OTP
for Reliable Authentication
Improve authentication success with automated voice verification when SMS OTP delivery is unavailable or delayed.
What is Voice OTP?
Voice OTP is an authentication method that delivers one-time passwords through an automated voice call instead of SMS. It is commonly used as a fallback when SMS delivery is delayed, blocked, or unavailable.
Businesses use Voice OTP to improve verification success while reducing customer login failures.
- Coverage: 220+ countries
- Delivery method: Outbound automated voice call (TTS or prerecorded)
Why businesses use Voice OTP
Voice delivery modes
SureM ACS supports two voice delivery modes.
Upload custom voice files for brand-consistent OTP delivery in specific languages.
Dynamically generated voice prompts for flexible multilingual OTP delivery without pre-recording.
Supported languages
Supported languages include Korean, English, Chinese (Mandarin), Japanese, Spanish, and additional languages available upon request.
Voice OTP vs SMS OTP
| Feature | SMS OTP | Voice OTP |
|---|---|---|
| Delivery | SMS | Voice call |
| Network dependency | High | Lower |
| Read required | Yes |  No |
| Accessibility | Medium | High |
| Fallback | No | Yes |
SMS-to-Voice fallback flow
How ACS triggers automatically when SMS OTP is not delivered.
- SMS OTP sent to recipient
- If undelivered within X seconds → ACS triggers automatically
- Automated voice call placed to same number
- OTP read aloud via prerecorded or TTS prompt
- Answer status confirmed to client system
Authentication flow
Use cases
Global coverage & accessibility
Voice OTP is especially valuable where SMS is unreliable, and for users who benefit from an audio channel.
Countries with carrier filtering, countries with unreliable SMS delivery, roaming users, international visitors, and virtual numbers.
Supports users who are visually impaired, older adults, and users with lower text literacy.
Frequently asked questions
What is Voice OTP and how is it different from SMS OTP?
Voice OTP (also called ACS — Auto Calling Service) delivers a one-time password via an automated outbound voice call instead of a text message. When a user triggers authentication, the system places an automated call to their mobile or landline number and reads the OTP aloud using prerecorded audio or TTS (Text-to-Speech).
The key difference from SMS OTP is delivery channel reliability. SMS OTP can fail due to spam filtering, carrier-level blocking, roaming restrictions, or unavailability in certain regions. Voice OTP bypasses these SMS-specific constraints by routing through voice call infrastructure, making it particularly effective as a fallback for international users in regions where SMS delivery is inconsistent.
Which countries does SureM Voice OTP (ACS) support?SureM Voice OTP supports delivery to 220+ countries and territories worldwide. Coverage includes Asia-Pacific, Europe, the Americas, the Middle East, and Africa. Voice OTP is especially valuable for markets where SMS delivery rates are lower due to carrier filtering or regulatory restrictions — including China, India, Southeast Asia, and parts of the Middle East. Contact SureM for country-specific availability and rate information.
Does SureM ACS support TTS (Text-to-Speech) or only prerecorded audio?SureM ACS supports both delivery modes:
- Prerecorded audio: Custom voice files uploaded by the client for brand-consistent OTP delivery. Ideal when consistent pronunciation and tone are required in specific languages.
- TTS (Text-to-Speech): Dynamically generated voice prompts that read the OTP code aloud without requiring pre-recorded files. Supports multilingual delivery including Korean, English, Chinese, Japanese, and additional languages.
For most international OTP use cases, TTS is recommended for its flexibility. Prerecorded audio is preferred for high-volume, single-language deployments where audio quality consistency is critical.
Can Voice OTP be used as automatic fallback when SMS delivery fails?Yes. SureM ACS is designed to integrate with SMS OTP flows as an automatic fallback. The typical fallback sequence works as follows:
- SMS OTP is sent to the recipient’s mobile number
- If the SMS is undelivered within a defined timeout period, ACS triggers automatically
- An automated voice call is placed to the same number
- The OTP is read aloud via prerecorded prompt or TTS
- Call answer status and delivery confirmation are returned to your system via API
This fallback mechanism is particularly effective for international user registration, login verification, and high-value transaction confirmation where SMS failure would otherwise block the user flow.
What languages are supported for Voice OTP prompts?SureM ACS supports multilingual voice prompt delivery including Korean, English, Mandarin Chinese, Japanese, and Spanish. Additional languages are available upon request depending on destination country and volume requirements. For TTS-based delivery, language is typically matched to the destination country’s primary language automatically.
What is the typical delivery time for a Voice OTP call?Voice OTP calls are typically connected within a few seconds of the API request for domestic and major international destinations. Delivery time varies by destination country, local carrier routing, and network conditions. SureM provides real-time call status updates via API so your system can track answer status, call duration, and delivery confirmation in real time.
What are the most common use cases for Voice OTP?SureM Voice OTP (ACS) is commonly used for:
- Foreign member registration: Verifying international users who may not receive SMS reliably on non-Korean numbers
- Login and account access: Two-factor authentication (2FA) for users in regions with low SMS delivery rates
- Password reset: Secure identity confirmation for account recovery flows
- High-value transaction confirmation: Additional verification layer for financial transactions, large orders, or sensitive account changes
- SMS OTP fallback: Automatic escalation to voice when SMS delivery fails or times out
Authentication & related
Need a backup when SMS
authentication fails?
Deploy Voice OTP with SureM — REST API, global coverage, multilingual voice, and enterprise SLA.
SureM is a Korea-based business messaging infrastructure provider with 26 years of experience. SureM helps global messaging partners, foreign-invested companies in Korea, and Korean enterprises deliver SMS, LMS, MMS, RCS, Kakao, WhatsApp Business, Voice OTP, and international messages through reliable local network connectivity and global messaging reach.