Fonoster: The open-source alternative to Twilio

Fonoster is researching an innovative Programmable Telecommunications Stack that will allow businesses to connect telephony services with the Internet entirely through a cloud-based utility.

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The most notable features of Fonoster are:

• Multitenancy
• Easy deployment of PBX functionalities
• Programmable Voice Applications
• NodeJS SDK
• Support for Amazon Simple Storage Service (S3)
• Secure API endpoints with Let's Encrypt
• Authentication with OAuth2
• Authentication with JWT
• Role-Based Access Control (RBAC)
• Plugins-based Command-line Tool
• Support for Google Speech APIs

A Voice Application is a server that controls a call's flow. A Voice Application can use any combination of the following verbs:

• Answer - Accepts an incoming call
• Hangup - Closes the call
• Play: Takes a URL with a media file and streams the sound back to the calling party
• PlayDtmf - Takes a DTMF sequence and plays it back to the calling party
• Say - Takes a text, synthesizes the text into audio, and streams back the result
• Gather - Waits for DTMF or speech events and returns back the result
• SGather - Returns a stream for future DTMF and speech results
• Stream - Creates a bidirectional stream to send and receive audio from a caller
• Dial - Passes the call to an Agent or a Number at the PSTN
• Record - It records the voice of the calling party and saves the audio on the Storage sub-system
• Mute - It tells the channel to stop sending media, effectively muting the channel
• Unmute - It tells the channel to allow media flow

Voice Application Example:

const VoiceServer = require("@fonoster/voice").default;
const { 
 GatherSource, 
 VoiceRequest, 
 VoiceResponse 
} = require("@fonoster/voice");
new VoiceServer().listen(async (req: VoiceRequest, voice: VoiceResponse) => {
 const { ingressNumber, sessionRef, appRef } = req;
 await voice.answer();
 await voice.say("Hi there! What's your name?");
 const { speech: name } = await voice.gather({
 source: GatherSource.SPEECH
 });
 await voice.say("Nice to meet you " + name + "!");
 await voice.say("Please enter your 4 digit pin.");
 const { digits } = await voice.gather({
 maxDigits: 4,
 finishOnKey: "#"
 });
 await voice.say("Your pin is " + digits);
 await voice.hangup();
});
// Your app will live at tcp://127.0.0.1:50061 
// and you can easily publish it to the Internet with:
// ngrok tcp 50061

Everything in Fonoster is an API first, and initiating a call is no exception. You can use the SDK to start a call with a few lines of code.

Example of originating a call with the SDK:

const SDK = require("@fonoster/sdk");
async function main(request) {
 const apiKey = "your-api-key";
 const apiSecret = "your-api-secret"
 const accessKeyId = "WO00000000000000000000000000000000";
 const client = new SDK.Client({ accessKeyId });
 await client.loginWithApiKey(apiKey, apiSecret);
 const calls = new SDK.Calls(client);
 const response = await calls.createCall(request);
 console.log(response); // successful response
}
const request = {
 from: "+18287854037",
 to: "+17853178070",
 appRef: "3e61ecb7-a1b6-4a93-84c3-4f1979165bca",
 // Optional metadata to be sent to the Voice Application
 metadata: {
 name: "John Doe",
 message: "Please call me back."
 }
};
main(request).catch(console.error);

To get started with Fonoster, use the following resources:

• Deploying Fonoster with Docker
• Guide for Early Access User
• Getting started with Fonoster
• How we created an open-source alternative to Twilio and why it matters

Please give it a star if you like this project or plan to use it. Thanks 🙏

For bugs, questions, and discussions, please use the Github Issues

For contributing, please see the following links:

• Contribution Documents
• Contributors

We're glad to be supported by respected companies and individuals from several industries.

Find all our supporters here

Become a Github Sponsor

• Pedro Sanders

Copyright (C) 2025 by Fonoster Inc. MIT License (see LICENSE for details).