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Two-factor authentication

Verify users by reaching their mobile device with SMS or voice codes: easily adding a layer of security.
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A single API provides the full 2FA solution, from authentication management to message automation, spanning SMS and Voice.


Just give us a phone number and we’ll take care of the rest. We generate the codes, localize, use the fastest channel available, even fall back from SMS to voice when needed.


Unlike other 2FA methods that may require special hardware or an authenticator app, our solution works with any phone number.


Pay only for successful conversions or per verification request.

How two-factor authentication works

Four asterisks

Intercept the login

A user logging into an account first confirms their username and password, but to be certain that the user is who they say they are, our Verify API gets involved.

Check mark in shield

Our Verify API reaches out

We send a one-time code — via SMS, voice, or push notification — to the phone associated with that username and password.

Pictogram of four stars and an check mark indicating successful verification.

The account owner confirms

When the code arrives on the phone belonging to the user associated with the account, the owner keys in the short verification code into the input box presented by the app.

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Allow account access

Your app verifies that the entered code matches the code that was sent, confirming that the person attempting to access the account has the phone linked to that account. Your app can then give full access to the user.

Programmable elements used in this solution

It sounds easy, but...

Application complexity

Building a 2FA mechanism from the ground up is hard. There’s much more to 2FA than sending messages. You have to generate, store, and expire secure codes, as well as design and build a system to relate user identities to devices.

One channel is not enough

Once you’ve obtained a user’s phone number, you want to be able to verify them, no matter the line type. This means you have to first detect the line type (mobile or landline), and then deliver a message via either SMS or voice, depending on the type of device.

Deliverability is critical

Because the user is in the process of logging in, timeliness of message delivery is critical. Your 2FA solution falls apart if you can’t get the required code to the user within a few seconds.

Development cost

To bring 2FA to your mobile app, you may also have to design and build the UI for each device, increasing development time and expense.

Why choose Vonage APIs for two-factor-authentication?

Comprehensive solution

We designed the Verify API to make it simple to implement. Your application simply gives us a phone number and we take care of the rest. We’ve got you covered for SMS, voice, and SMS to voice failover scenarios. We even make sure the messages comply with local regulations so they are not filtered by the carriers. Our comprehensive backend provides the secure code and identity management capabilities, all behind a simple to use API.

Highest deliverability

Our many direct-to-carrier relationships around the world, combined with our proprietary Adaptive Routing algorithm, allow us to work in real-time to find the best routes for your messages. Add to that our Compliance Engine, which knows how and when to deliver messages according to country and carrier requirements, and you have the industry’s highest deliverability rates.

Pay only for success

We priced Verify in the way that makes the most sense to you and is easy to ROI -- you pay only one flat fee for successful conversions.

Pre-built user experience

The Verify SDK for mobile apps includes a customizable user experience framework that lets you build the 2FA UI into your app very quickly. You can implement cross-device logins and specify expiry times to re-authenticate users without building a complex authentication back-end.

"We would not be able to achieve virality without a reliable, scalable phone number authentication solution, and we would not have been able to do that without [Vonage APIs] and the Verify API."

Alexis Bonillo Co-Founder & COO, Zenly

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