Reverse OTP vs. SMS OTP: which direction wins?
With an SMS OTP, you send a code and the user types it in. With reverse OTP, your page shows the code and the user sends it to you. This guide compares both step by step: flow, delivery, security and the cases where the classic SMS code is still the better choice.
- Two directions, one goal
- Where SMS OTP gets stuck
- When SMS OTP fits better
Christian Schätzlein, Founder and developer of Sendity
3 min read
Where SMS OTP gets stuck
The five steps of an SMS code
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Type number Typos, wrong country code User
The code goes to a number that does not belong to the user.
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Send code Sender ID rules per country You
Many countries require registered sender names or templates.
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Delivery Delays, filters, roaming Network
Users wait, tap resend, and you pay twice.
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Type code Autofill or typing User
Smooth with autofill on the same phone, tedious on a computer.
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Check Code matches You
Now the number counts as verified.
Sign-ups by message. No form, no code.
Sendity is a sign-up widget for your website. Instead of filling in a form, your visitors send one short message via WhatsApp or email. Sendity checks who it came from and hands your site the name and the confirmed phone number or email address.
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One message instead of a form Visitors tap send in WhatsApp or their email app. Nothing to type, no confirmation email to click.
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Confirmed by the message itself The contact comes from the sender of the message, so typos and made-up addresses never reach your list.
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Pay per confirmed contact Start with 1,000 free credits, no credit card. After that, prepaid packages with no subscription.
Both prove that a phone number is in the user’s hands. They differ in who sends, who types and who pays for failures.
SMS OTP
The user types their number, you send a code by SMS, the user types the code back. Every phone can receive it, and phones can fill in the code automatically.
Reverse OTP
Your page shows a code, the user sends it in a prepared WhatsApp message, by email or by SMS. The number comes from the sender, so nothing has to be typed.
Delivery
SMS codes depend on carriers, filters and sender rules in each country. With reverse OTP, nothing is delivered to the user; their message only has to reach you, usually over the mobile internet.
Security
Reverse OTP leaves nothing to intercept on the way to the user and nothing to pump. Phishing hits both: a fake page can capture a typed code or show a real one to send.
When SMS OTP fits better
In a native app with code autofill, for users without mobile internet or messenger, and where switching to another app would break the flow.
Reverse OTP step by step.
The same goal in three steps, with the direction turned around.
Your page shows a code
Short-lived and tied to this request. On a computer as a QR code.
The user sends it
The message is prepared; the user only taps send.
You receive the verified number
Taken from the sender, as a signed result for your backend.
SMS OTP or reverse OTP?
The differences that matter in practice.
| Aspect | SMS OTP | With Sendity |
|---|---|---|
| Who sends | You | The user |
| What the user types | Number and code | Nothing |
| On a computer | Read the code off the phone and type it | Scan the QR code, send on the phone |
| Failed attempts | Every SMS is billed | Cost nothing |
| Without smartphone | Works on any phone | By SMS or email, if you allow it |
What turning the direction changes.
Fewer places where a verification can fail.
Nothing to type
No number, no code, no typos.
No waiting for an SMS
Carrier delays and filters do not hold up the code.
No sender registration
You do not send, so country rules for SMS senders do not apply.
Live on your page in minutes.
Sendity runs as a compact widget on your page. Pick the path that fits your website.
Reverse OTP vs. SMS OTP: common questions.
Is reverse OTP a standard?
No formal one. The term describes the reversed direction and is used by several providers, each with its own implementation.
What about SMS code autofill?
On the same phone it makes SMS OTP very smooth. It does not help on a computer, and it does not prevent delays or costs.
Can I combine both?
Yes. For example reverse OTP on the website and SMS OTP inside a native app with autofill.
Does the user pay for sending?
WhatsApp and email use the existing data plan. With the SMS fallback, the user pays their usual SMS rate.
Keep reading.
Reviewed for accuracy in by Christian Schätzlein.
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