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The AI that answers the phone: an incoming call answered by the Front desk agent, with the agent and the caller talking. The AI that answers the phone: an incoming call answered by the Front desk agent, with the agent and the caller talking. Fonoster is an open-source platform for building, testing and running voice agents on real phone lines. Your agent answers calls on your numbers, makes calls, transfers to people and keeps a record of every conversation. Use Fonoster Cloud, or run Fonoster on your own servers. Your agents and code work the same on both.

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Two ways to build

Autopilot

Fonoster runs the agent for you. Write the prompt, pick the voice and the language model, and give it tools. You don’t run a server.

Streams

You run the agent on your own WebSocket server. Write it with the Voice SDK for Node.js or Python, or connect a Twilio or Pipecat bot you already have.

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What every call gets

Both ways share the same phone layer, so you don’t build it yourself:
  • Numbers and SIP trunks from the carrier you choose
  • Transfers to a person, cold or warm
  • DTMF: read and send keypad tones
  • Answering-machine detection, most useful on outbound calls
  • Recording, with a consent prompt when you need one
  • A signed record of every call, with its transcript and recording
  • Webhooks when calls start and end

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Start here

Quickstart

Create an Autopilot agent and call it.

How Fonoster works

Learn how workspaces, numbers and applications fit together.
To run Fonoster on your own servers, see Self-hosting. To change Fonoster itself, see Hacking Fonoster. The SDK and CLI references are one click away in the top bar.