We've created a smart voicemail system using Twilio Functions, JavaScript, and the Google Calendar API. The system checks if the user is available or busy by sending a Freebusy query to the Google Calendar API. If the user is unavailable, it sends a message saying so and provides an option to leave a voicemail. When a voicemail is recorded, the system sends a notification to the user's personal number with a link to the voicemail audio. The system also uses TwiML to forward calls to the user's mobile number when they are available or to send a recording of the missed call as a message.