Twilio Add-ons are pre-integrated third-party services that can be used with Twilio applications to enhance the responses received from Twilio by including additional data from Twilio's partners. To use an add-on, first enable it in the Twilio Console and then select the recording method and set a callback URL as needed. The add-on data will be included in an "AddOns" field on HTTP requests made to your application. In this guide, we used the IBM Watson Speech to Text Add-on to convert Twilio audio recordings into text using machine intelligence. The generated transcription is pushed to the application through a callback from Twilio, allowing developers to access and utilize the additional data in their PHP applications.