"In this post, we explored streaming an audio file into an in-progress phone call using the Vonage Voice API and Ruby gem. To achieve this, we created a Sinatra web server that serves four routes: three GET requests to initiate the phone call, provide NCCO instructions, and stream a silent file, and a POST request to stream audio once the call has been answered. We utilized the dotenv gem to store our Vonage account credentials as environment variables, ensuring they are not committed to source control. With approximately 40 lines of code, we successfully created a fully functioning web server that can call any phone number and play streaming audio into that call."