A birthday time capsule system was created using Go and Vonage's Voice API, allowing well-wishers to leave voice recordings for a birthday person at a specified date and time. The system consists of several components, including a virtual phone number, an Ngrok tunnel, a Vonage application with webhooks, and a database to store the recorded audio files. When a call is made to the virtual phone number, the system answers the call and instructs the caller to leave a message. The messages are then stored in the database and played back to the birthday person at a later date using an outbound call from Vonage's Voice API. The system also includes functionality to play multiple audio files in sequence, with each file being played after the previous one is completed.