How to Connect Your Twilio Agent to External APIs

Twilio’s tutorial shows .NET developers how to build a voice-enabled AI agent that uses Conversation Relay, OpenAI tool calling, and an external REST API to provide real-time information and perform actions. The example creates an ASP.NET Core application in C# .NET 9, configures a Twilio voice webhook and WebSocket endpoint, stores an OpenAI API key in environment variables, and connects incoming speech prompts to an OpenAI-powered conversational workflow. Using the Cat Facts API as a simple unauthenticated demonstration, the agent is instructed to call a defined tool rather than invent information, retrieves a live fact through HTTP, and streams a natural-language response back to the caller. The tutorial also explains testing the application with ngrok and a Twilio phone number, notes common troubleshooting steps for webhook URLs and API credentials, and emphasizes that the same pattern can be expanded to authenticated APIs for customer data, inventory, calendars, appointments, and other practical voice-agent capabilities.

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