This article demonstrates how to validate phone numbers in a Spring Boot application using the Twilio Lookup API, providing a more reliable and comprehensive validation process than relying solely on regular expressions or basic string checks. By creating a custom annotation `@ValidPhoneNumber` and implementing a `ConstraintValidator` class that uses the Twilio API to verify the phone number's validity, developers can ensure that only real phone numbers are accepted in their application, reducing the risk of spam or incorrect information. The article provides step-by-step instructions on how to implement this validation process, including creating the annotation, validator, and configuring the Twilio client, as well as testing and wrapping up the demo application.