Nixflex - AI Voice Platform for Building AI Phone Agents

Who this is for

There are two ways to listen to a live call:

Business owners - open the dashboard, see your live calls, tap Listen. No code.
Developers - build listening into your own product with the API below. Your customers listen inside your app, on your brand.

Only inbound calls can be listened to, and only on numbers that have monitoring enabled. Everything is isolated per account: you can only ever see and hear calls on numbers you own.

Listening from the dashboard

Open Live Monitor in the dashboard, pick a number, and any live call on it appears in the list. Tap Listen and you hear both sides in real time - the caller and the AI - in sync. When the call ends, the player tells you. That is all a business owner needs. The rest of this page is for developers.

Building listening into your own app

The flow has three steps: list the live calls, open the listen socket, then join the audio stream.

1. List live calls

curl "https://api.nixflex.com/v1/monitor/calls" \
  -H "Authorization: Bearer nxf_xxx:nxfs_xxx"

{
  "calls": [
    {
      "call_id": "CAxxxxxxxxxxxxxxxx",
      "number": "+447446466847",
      "caller": "+447453573770",
      "direction": "inbound",
      "state": "LISTENING",
      "started_at": 1753970000000
    }
  ]
}

Only live calls on numbers your key owns and that have monitoring enabled are returned. Poll this to show a live-calls list in your UI.

Optional query parameter ?source=dashboard narrows the list to numbers added through the dashboard. Without it, every monitored number on your key is included - which is what you want when you manage numbers through the API.

2. Open the listen socket

Connect a WebSocket with the call id and your API key:

wss://api.nixflex.com/monitor/listen?callId=CAxxxxxxxxxxxxxxxx&key=nxf_xxx:nxfs_xxx

The engine verifies the call is live, the number is yours, and monitoring is on. On success you receive one message with everything needed to join the audio:

{
  "event": "connected",
  "callId": "CAxxxxxxxxxxxxxxxx",
  "livekit": {
    "url": "wss://....livekit.cloud",
    "token": "eyJhbGci...",
    "room": "call-CAxxxxxxxxxxxxxxxx"
  }
}

If a check fails, you receive an error message and the socket closes:

Code	Meaning
`missing_call_id` / `missing_key`	A required query parameter was not sent
`call_not_found`	That call is not live right now
`invalid_key` / `key_inactive`	The API key was not recognised or is disabled
`not_your_number`	The call is not on a number this key owns
`monitor_disabled`	Monitoring is not enabled for this number
`audio_unavailable`	The audio stream could not be started - try again

Keep the socket open while listening. It carries control messages - most importantly:

{ "event": "call_ended", "reason": "caller_hangup" }

3. Join the audio

Audio streams over managed WebRTC, not the WebSocket. Join with the standard livekit-client library using the url, token, and room from the connected message:

import { Room, RoomEvent } from 'livekit-client';

const room = new Room();
room.on(RoomEvent.TrackSubscribed, (track) => {
  // Two audio tracks arrive: the caller and the AI.
  document.body.appendChild(track.attach());
});
await room.connect(livekit.url, livekit.token);

The call is published as two separate audio tracks - customer-audio (the caller) and ai-audio (the agent) - which your player mixes automatically when both are attached. Two tracks means you could also show separate volume meters, or mute one side. The token is listen-only: it cannot publish audio into the call, and it only works for that one room.

Behaviour and guarantees

Zero effect on the call. The audio stream only opens while someone is actually listening. If anything in the monitoring path fails, the call continues normally - the caller never notices.
Isolation. Ownership is checked when listing calls and again when the listen socket connects. One account can never hear another account’s calls.
Multiple listeners can join the same call at once.
Listen-only today. Speaking into the call and taking over from the AI are planned for a later stage.

Live Monitor overview - what monitoring is, pricing, enabling it from your app
Live monitor toggle (API) - turn monitoring on or off per number

Get started

Core concepts

In-call actions

Features

SMS

Reference

Listening to live calls

Who this is for

Listening from the dashboard

Building listening into your own app

1. List live calls

2. Open the listen socket

3. Join the audio

Behaviour and guarantees

​Who this is for

​Listening from the dashboard

​Building listening into your own app

​1. List live calls

​2. Open the listen socket

​3. Join the audio

​Behaviour and guarantees

​Related

Who this is for

Listening from the dashboard

Building listening into your own app

1. List live calls

2. Open the listen socket

3. Join the audio

Behaviour and guarantees

Related