Two-factor authentication (2FA) is a security process that requires a user to provide two different authentication factors to access a system, network, or application. In this tutorial, we covered how to add 2FA to an iOS application using the Nexmo Verify SDK, which provides SMS and TTS verification, as well as biometric fingerprint or retina scan verification. The app uses a virtual phone number and a Back4App backend for user login and data storage. We also added a new view controller for verification and implemented logic for SMS verification on login, PIN code verification, and transfer functionality with 2FA.