When creating an account, you ask a would-be user to enter their phone number in your app or website. Next, you deliver a PIN via SMS to the registered number. If the number is legitimate, the user will receive the SMS and enter the PIN. If the PIN matches, you are assured of the association between that user and that phone number. This approach to validation sounds relatively simple, but practically speaking, building and maintaining a phone verification system is fraught with challenges, especially when onboarding users at global scale.