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OTP by SMS or WhatsApp: which should you use for login codes?
SMS reaches every phone, with or without data. WhatsApp is cheaper in Sri Lanka and feels more private, but needs the app and an internet connection. Most businesses get the best result by using one first and the other as a fallback.
The short answer
Use SMS as your safe default for login, payment and sign-up codes, because it reaches every handset. Use WhatsApp authentication when most of your users are active on WhatsApp and you want a lower cost per code. For important flows, send on one channel and fall back to the other if the code is not delivered in a few seconds.
SMS OTP and WhatsApp authentication side by side
| SMS OTP | WhatsApp authentication | |
|---|---|---|
| Reach | Any phone with a signal | Users with WhatsApp and a data connection |
| Sender identity | Your brand as a sender ID where supported | Your verified WhatsApp business profile |
| Delivery status | Delivery report by message ID | Sent, delivered and read status |
| Cost in Sri Lanka | Set by your SMS provider | US$0.002 per authentication message from Meta, plus platform fee |
| Roaming and abroad | Depends on the operator and route | Works over data, but Meta lists US$0.144 for codes sent from abroad to Sri Lankan numbers |
| Setup | Sender ID and a route | WhatsApp Business API and an approved authentication template |
The fallback pattern
- Send the code on your primary channel and start a short timer, for example 20 seconds.
- If delivery is not confirmed, send the same flow on the second channel.
- Accept the newest code only, and invalidate the old one.
- Show a "resend" button after the timer, with a limit on resends.
Unecast sends SMS OTP on a high-priority transactional route and WhatsApp authentication from the same account, so one integration can cover both.
Security basics for any OTP
- Expire codes fast. Five minutes is a common limit.
- Make them single-use. A used or replaced code must stop working.
- Limit attempts. Lock or slow down after a few wrong entries, and limit sends per number.
- Say what the code is for. Include your brand name and a warning not to share it.
- Know the limits. OTP by SMS or WhatsApp can be phished or targeted by SIM swapping. For high-value actions, add another check.
What the integration looks like
With Unecast, you send an OTP with one POST request and check its status with one GET, using a bearer token. A balance API lets your system top up before it runs out, so codes never fail for lack of credit. See the OTP SMS page and the developer page for details.
Frequently asked questions
Is SMS or WhatsApp better for OTP?
SMS reaches every phone and is the safe default. WhatsApp can cost less and works well when your users are active on it. Many businesses use one first and the other as a fallback.
How much does a WhatsApp OTP cost in Sri Lanka?
Meta charges about US$0.002 per authentication message within Sri Lanka, plus your platform fee. Codes sent from abroad are listed at about US$0.144.
How long should an OTP last?
Five minutes is a common limit, and each code should work only once.
What is a sender ID?
It is the name shown as the sender of an SMS, such as your brand, instead of a phone number. Rules vary by country and operator.
Can I send OTP by SMS and WhatsApp from one account?
Yes. On Unecast, SMS OTP and WhatsApp authentication run from the same account.
Keep reading
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