Guide for verification

Phone verification methods: six ways compared.

Verifying a phone number does not have to mean an SMS code. This guide compares six methods, from SMS code, voice call and flash call to silent authentication, WhatsApp code and reverse OTP: how each one works, what it needs and where it falls short.

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Six methods at a glance
What each one needs
Which fits your use case

Christian Schätzlein, Founder and developer of Sendity 3 min read

verification-methods

Six methods, one goal

Method, how it works, what the user does

SMS code You send a code, the user types it Types code

Works on every phone; delivery and costs depend on carriers.
Voice call A call reads the code out Types code

Fallback for landlines and poor SMS delivery.
Flash Call A missed call, the number is the code App reads it

Automatic only in Android apps; carriers increasingly block or charge it.
Silent Auth The carrier checks the SIM in the background Nothing

Only over mobile data and with supporting carriers.
WhatsApp code You send a code via WhatsApp Types code

Needs a verified business and an approved template at Meta.
Reverse OTP The user sends the code shown Taps send

Nothing outgoing; the number comes from the sender.

What is Sendity?

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.
Confirmed by the message itself The contact comes from the sender of the message, so typos and made-up addresses never reach your list.
Pay per confirmed contact Start with 1,000 free credits, no credit card. After that, prepaid packages with no subscription.

Either you send something to the user, the network checks in the background, or the user sends something to you.

SMS code and voice call

The classics: a code by SMS or read out in a call, typed in by the user. They reach every phone, cost per attempt and are exposed to pumping and interception.

Flash call

A call that rings once; the caller’s number contains the code. An Android app can read it automatically, on iPhones users have to type the digits. Carriers increasingly treat flash calls as abuse.

Silent authentication

Over the mobile data connection, the carrier confirms that the SIM belongs to the number, without any user action. It does not work over Wi-Fi, and availability depends on carriers and aggregators.

WhatsApp code

Like SMS, but via WhatsApp: encrypted and independent of SMS delivery. You need a verified business and an approved authentication template at Meta, and each message costs.

Reverse OTP

Your page shows the code, the user sends it via WhatsApp, email or SMS. You send nothing, so there is nothing to pump, and the user types nothing.

How it works

Choosing a method.

Three questions narrow it down.

Website or app?
Flash call and silent authentication need an app or a mobile data connection; on a website, SMS, WhatsApp and reverse OTP remain.
Who should pay for failures?
Outgoing methods bill every attempt. Reverse OTP bills only successful verifications.
Who must you reach?
For users without smartphones, keep SMS or a voice call as a fallback.

Compared

Outgoing code or reverse OTP?

The four outgoing methods share most of their limits.

Aspect	SMS, call, flash call, WhatsApp code	With Sendity
Who sends	You	The user
Cost per failed attempt	Yes	None
Pumping risk	Yes	None
Typed by the user	Number, usually also the code	Nothing

Benefits

Where reverse OTP stands out.

Among the methods that work on a plain website.

Works on any website

No app, no carrier contract, no SMS sender registration.

Pay per success

Failed and abandoned attempts cost nothing.

Email in the same widget

Where a phone number is not needed, visitors verify their address instead.

Integration

Live on your page in minutes.

Sendity runs as a compact widget on your page. Pick the path that fits your website.

WordPress plugin Add Sendity to any page as a block. No code needed. Read the guide Laravel package Check the signed result on your server in a few lines. Read the guide Any other website Paste one snippet into your page and match the widget to your brand. Read the guide

FAQ

Verification methods: common questions.

Which method is the most secure?

For proving a number, silent authentication and reverse OTP leave the least to intercept. Against phishing, none of them protects as well as a passkey.

Which is the cheapest?

That depends on your success rate: outgoing methods bill every attempt. Our cost guide shows how to work out your real price per number.

Why not silent authentication everywhere?

It needs mobile data instead of Wi-Fi, a supporting carrier and usually an aggregator contract. As one layer among others it can be very convenient.

Can I combine methods?

Yes, and many do: for example reverse OTP on the website, SMS as a fallback for users without WhatsApp.

Guide Reverse OTP vs. SMS OTP: which direction wins? Flow, delivery and security step by step, and when the classic SMS code is still the better fit. Guide Phone number verification: what does it cost? List prices of Twilio Verify, otp.dev, phone-verif and Sendity, the costs outside them and how to work out yours. Guide Is SMS verification still secure? Phishing, SIM swap, malware and SS7, what SMS is still good enough for and where reverse OTP helps and where it does not. Product Phone number verification Security checks, SMS fallback and costs.

Reviewed for accuracy in November 2026 by Christian Schätzlein.

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