ASP.NET Core Identity provides basic phone number validation using a regular expression, but this can be improved by integrating Twilio's telephony features. The author demonstrates how to validate phone numbers with Twilio's Lookup API, which uses the libphonenumber library to check for validity and returns additional information such as carrier and type of the phone number. To improve the robustness of phone number validation, users must choose a country code when registering their phone number. The author shows how to add a dropdown list of countries using a JSON file and how to validate phone numbers with Twilio's Lookup API. The process also includes checking if the phone number can receive SMS messages and saving the formatted E.164 number to the Identity user's profile.