The international telephone input JavaScript plugin, developed by Twilio, is a tool that facilitates the collection of phone numbers in web applications. It enables developers to easily integrate phone number input into their forms, making it easier to collect and validate phone numbers for various use cases such as phone verification, SMS 2FA, and requesting callbacks from customer service. The plugin offers quick deployment options through Twilio's Quick Deploy, allowing developers to set up an international telephone input in under 5 minutes. This enables the creation of secure and user-friendly sign-up forms that can handle a wide range of phone number formats and currencies.