A developer can build an AI voice assistant on WhatsApp using Python, Whisper API, ChatGPT API, and Twilio by setting up a backend with FastAPI and SQLAlchemy to create a PostgreSQL database. The assistant will transcribe voice notes from WhatsApp users using the Whisper API and generate intelligent responses via the OpenAI ChatGPT API. To test the chatbot, a Twilio-approved webhook must be configured to receive incoming messages from WhatsApp users, which are then processed by the FastAPI application. The final result is an AI-powered chatbot that can understand voice notes and provide relevant responses to users on WhatsApp.