The Flask-WTF package makes it straightforward and fun to work with forms in Flask, but sometimes you need to include fields that fall outside of the standard primitive field types. In this article, a phone number input field for Flask forms is built using Flask-WTF and Bootstrap-Flask, which provides an easy way to create forms based on the Bootstrap 4 CSS/JavaScript framework. The form includes a dropdown that lists all the countries with cute little flag icons and their corresponding numeric country codes, once a country is selected, the content portion of the field shows a phone number template that helps the user know how to enter their number correctly. JavaScript to the rescue! The intlTelInput library is used to implement this fancy phone input field. The form rendering uses a hybrid approach by rendering the form with the Bootstrap-Flask macro and then dynamically inserting a JavaScript-based phone input field in the correct position, which is an effective way to make the field invisible but still visible for editing purposes.