A failed OTP can mean a lost transaction, a frustrated user, or even a failed sign-in during a critical moment. But not all OTP delivery methods are built for challenging environments—especially in areas where internet access is limited, data is patchy, and mobile connectivity drops in and out. In such regions, which method really stands up: Voice SMS or USSD?
This isn’t just a technical comparison—it’s a practical decision with direct impact on user experience, financial transactions, and even compliance. Whether you’re a fintech app verifying logins or a telco pushing SIM registration alerts, choosing the right OTP delivery method matters.
Let’s break down both technologies and compare how they stack up for OTP delivery, especially where traditional SMS and email don’t cut it.
Understanding OTP Delivery and Why It Fails
But what do we mean by OTP delivery? Well, let us find out before we begin the comparison. One-Time Passwords (OTPs) are temporary codes which are forwarded to users to authenticate their identity during log-in, payments or account change. The OTP must arrive fast, be accessible, and not rely heavily on data connectivity or smartphone sophistication.
In low-connectivity regions—think rural India, remote Africa, or underserved areas in Southeast Asia—traditional SMS delivery often fails due to:
- Poor mobile signal
- Congested networks
- Device limitations
- SIM registration issues
- SMS spam filters
That’s where alternative channels like Voice SMS and USSD step in.
What is Voice SMS for OTP Delivery?
Voice SMS leaves a recorded voice message on the telephone of the user. The user hears a code, rather than reads it. It’s ideal when the recipient has poor literacy levels, or when text messages fail to reach.
How Voice SMS Works:
- The OTP is read out through text-to-speech (TTS).
- The phone of the recipient is called.
- The user listens to a message: “Your OTP is six five nine zero.
- No need for mobile data or smartphone features.
Benefits:
- Works on all phones (feature and smartphones)
- Accessible to users with low literacy
- Useful in visually impaired user contexts
- Supports multiple languages and dialects
Challenges:
- The user must answer the call
- Call charges may apply for large-scale campaigns
- Calls can be missed or rejected
- May raise privacy concerns if overheard
What is USSD, And How can it be Beneficial In OTP Delivery?
Unstructured Supplementary Service Data (USSD) is a session messaging platform that is used in GSM networks. It is usually applicable in mobile banking, checking the balance, or airtime top-up. It does not rely on the internet and works even on the simplest feature phones.
How USSD Works for OTP:
- The user dials a code like *123# to initiate a session.
- The system prompts a response (e.g., “Enter your phone number”).
- OTP is displayed or entered within the session.
- It’s entirely interactive, without requiring incoming messages or calls.
Benefits:
- Works offline (no internet or app required)
- Interactive and instant
- Compatible with basic GSM phones
- Not stored on device, improving security
Challenges:
- Requires user initiation (dialing a USSD code)
- Some networks limit session duration
- UI experience can be clunky for long codes
- Not supported on CDMA networks or in some countries
Comparing Voice SMS vs USSD for OTP Delivery
Let’s assess these two methods side-by-side, focusing on key delivery KPIs for low-connectivity regions:
| Criteria | Voice SMS | USSD |
| Works without data | ✅ Yes | ✅ Yes |
| Compatible with feature phones | ✅ Yes | ✅ Yes |
| Literacy not required | ✅ Yes | ❌ No |
| User must initiate | ❌ No | ✅ Yes |
| Can be overheard | ⚠️ Possible | ✅ No (more discreet) |
| Fast delivery | ⚠️ Depends on call connection | ✅ Instant on session start |
| Language flexibility | ✅ Multi-language voice support | ⚠️ Limited text display |
| Cost to business | ⚠️ Higher per-call charge | ✅ Lower session cost |
| Suitable for silent mode | ❌ Missed if phone is silent | ✅ USSD always appears on screen |
Real-World Example: Voice SMS for OTP Delivery in Sub-Saharan Africa
In Sub-Saharan Africa, where literacy rates can be below 70% and mobile data is often unavailable, Voice SMS has shown significant success. Africala, a global messaging leader, deployed Voice SMS for a rural mobile banking service across Uganda and Tanzania. The results?
- 25% increase in successful logins
- 40% drop in failed OTP attempts compared to text
- Voice messages delivered in Swahili improved accessibility
According to Africala’s internal analytics, peak success occurred between 6 PM – 9 PM when users were off work and likely to answer calls.
Real-World Example: USSD for OTPs in South Asia
In India, fintech services like UPI payments and Aadhaar authentication use USSD for OTP flows when users don’t have mobile data access. One large microloan platform saw:
- OTP success rate of 92% via USSD compared to 65% via SMS
- 3x faster transaction completions
- Better fraud resistance due to session-based interaction
SMSala, a global player in omnichannel messaging, reports similar trends from Bangladesh and Nepal where telco infrastructure is limited but GSM support is widespread.
When Should You Choose Voice SMS?
Use Voice SMS for OTP delivery when:
- Your audience has lower literacy levels
- You need multilingual delivery
- Users don’t initiate the transaction (e.g., push-based alerts)
- You’re targeting visually impaired users
- Brand experience benefits from human voice (e.g., premium apps)
Industries Best Suited:
- Financial services (e.g., voice-based authentication)
- Healthcare (appointment verifications)
- Education portals (remote access to exam portals)
- E-commerce (voice confirmation for COD orders)
When Should You Use USSD for OTP Delivery?
Use USSD for OTPs when:
- User interaction is required (e.g., PIN entry)
- Network latency is an issue
- You want to avoid SMS delays
- Sessions need to be secure and not stored
- You want to keep operational costs low
Industries Best Suited:
- Fintech (USSD wallets, microloans)
- Government (SIM re-verification, Aadhaar)
- Telecom (airtime top-ups, plan upgrades)
- Utility services (bill payments, meter readings)
The Hybrid Approach: Combine Both for Better OTP Delivery
The best strategy may not be choosing one over the other—but combining both.
For instance:
- Use USSD as the default for OTP entry.
- If a user doesn’t respond within 30 seconds, trigger a Voice SMS.
- Allow user opt-in preferences (e.g., “Receive OTP via call” or “Enter via *123#”).
Global brands like Africala and SMSala are already deploying such multi-channel failover logic to increase OTP delivery success rates by over 98%.
Performance Metrics to Track OTP Delivery Efficiency
Whether you use Voice SMS or USSD, monitor these metrics for ongoing optimization:
- Delivery Rate: Was the OTP received?
- Conversion Rate: Was the OTP used to complete the transaction?
- Time to Verify: How fast does the user complete authentication?
- Cost Per OTP: How much are you spending on average?
- User Feedback: Are there complaints or drop-offs?
OTP Delivery in Low Connectivity: What the Data Says
According to a 2024 GSMA Mobile Connectivity Index report:
- 43% of Sub-Saharan Africa lacks reliable 3G
- 61% of Asia-Pacific rural populations use feature phones
- Only 24% of low-income users open text messages within 60 seconds
These stats make Voice SMS and USSD not just viable—but vital—for OTP delivery at scale.
Tools and APIs for Implementing Voice SMS and USSD
Providers like Africala and SMSala offer APIs that make it easy to integrate these methods with your app or backend:
- Voice SMS API: Send pre-recorded or TTS-based voice calls
- USSD Gateway: Set up custom USSD flows with OTP entry and real-time backend sync
Both platforms support high-throughput delivery, multi-language support, and global failover routing—critical for reaching users across continents.
Key Takeaways
- Voice SMS is user-friendly, spoken, and great for push-based OTP delivery.
- USSD is fast, secure, and requires the user to initiate the session.
- In low-connectivity regions, both methods outperform traditional SMS or email for OTP delivery.
- Brands like Africala and SMSala are setting benchmarks in hybrid delivery using voice and USSD.
- The ideal approach? Use both in tandem for higher reliability, lower cost, and a better user experience.
FAQs on Voice SMS vs USSD for OTP Delivery
Q1. Is USSD more secure than SMS for OTP delivery?
Yes. USSD operates over a session that isn’t stored on the phone, reducing risks from spyware or message storage.
Q2. Can Voice SMS work when the phone is on silent?
No. If the phone is silent or the user misses the call, they may not receive the OTP unless there’s a retry or fallback mechanism.
Q3. Which method is faster—Voice SMS or USSD?
USSD is generally faster since it doesn’t require call connection time. However, Voice SMS can be more accessible in specific contexts.
Q4. Are these methods expensive to implement?
Not necessarily. Voice SMS may have higher per-message costs due to telecom fees, while USSD is cheaper but may need integration with telecom operators.
Q5. Which are the areas where Voice SMS or USSD OTP delivery is most beneficial?
The highest returns are seen in the low internet penetration/ low literacy areas: Sub-Saharan Africa, South Asia, and rural Southeast Asia.
With a careful selection of Voice SMS, USSD, or a combination of both, brands can increase the reliability of OTP delivery even in the most problematic settings, without affecting the speed, user confidence, or cost effectiveness.