How to Stream Audio into a Call with Node-RED
Published on April 28, 2021

This is the fifth article in a series of “Getting Started with Nexmo and Node-RED” tutorials.

In the previous tutorials you've learnt about handling inbound calls and making outbound phone calls with the Nexmo Voice API. For these examples we've used text-to-speech, but sometimes a more human approach is needed. It's always a nice touch to be greeted by a human voice when calling a business, instead of the all too well known friendly neighbourhood robot. Also, time flies by easier when quality music is playing while you are holding the line.

In this blog post we'll change it up a bit, and you'll find out how to stream audio into a call. By the end of it, you'll be able to play an audio file to a caller and you'll know how to stream audio to the recipient of an outbound call.

Get the flows from the Node-RED library below or follow along!


Before getting started, you’ll need a few things:

  • Node.js and Node-RED installed on your machine

  • A way to expose your server to the internet. This either means you're running a hosted version of Node-RED, or in case you're developing locally, using a tunneling service like ngrok - get up to speed with this Getting Started with Ngrok in Node-RED tutorial

    Vonage API Account

    To complete this tutorial, you will need a Vonage API account. If you don’t have one already, you can sign up today and start building with free credit. Once you have an account, you can find your API Key and API Secret at the top of the Vonage API Dashboard.

Getting Your Credentials

In order to interact with the Voice API, you'll need to make note of a couple of things. Once you've created a Nexmo account, go to the dashboard to find your API key and secret.

Next, you'll need a Voice enabled virtual number. Go to Numbers > Buy numbers to get one.

Buy Number DashboardBuy Number Dashboard

Setting Up Your Node-RED Editor

First, you’ll need to install the runtime and editor. This could be done either on your local machine, on a Single Board Computer (eg Raspberry Pi), or a number of cloud-hosted options. This example will be using your local machine, so once you've installed Node-RED globally, just type the command below in your terminal to get started.


You can then access the Node-RED editor by pointing your browser at http://localhost:1880.

Once you have your editor open, you'll need to install the Nexmo nodes. You can do so under the Manage palette menu, by searching for the node-red-contrib-nexmo package and clicking install.

Install Node RedInstall Node Red

Now you should see all of the Nexmo nodes appear on the left side of your screen, among the other default nodes.

Exposing Your Local Server to the Internet

Next you'll have to expose your local server to the internet, so that Nexmo can access it. If you’re running Node-RED on a public webserver instead of your local machine, you can skip this stage.

Otherwise, a convenient way to do this is by using a tunneling service like ngrok.

First, you'll need to install the ngrok node. To do so, open up Manage palette from the hamburger menu in your Node-RED editor, search for the node-red-contrib-ngrok package and click install. After restarting your editor, the ngrok node should appear in the node palette.

ngrok manage palettengrok manage palette

The ngrok node takes the strings on or off as input to start/stop the tunnel, and outputs the ngrok host address as the msg.payload.

The easiest way to set this up is to wire two inject nodes as the ngrok node's input, one with the payload of the string on and the other with off. For easier use, you could also set the Name of these nodes accordingly in the node properties, so that it's clear what functionality they have.

Next, to display the host address in the debug sidebar, connect a debug node after ngrok.

As the last step before hitting deploy, open up the ngrok node properties and specify the port number. In case of Node-RED, the default value is 1880. The default ngrok Region is US but you can also set it to Europe or Asia. You can also add your authtoken for your ngrok account if you have one. Don't worry if you don't, just skip this step for now. The node will warn that it is not fully configured but this is not an issue.

ngrok propertiesngrok properties

And you're all set! Once you hit deploy and click on the on inject node's button, navigate to the URL displayed in the debug area (YOUR_URL for future reference) to find your Node-RED editor at a public address.

ngrok noderedngrok nodered

Event Webhook

If you'd like to receive events about the progress of your call, you can also setup an event webhook.

Connect a http input node to a http response node, as well as to a debug node, so that you can view your call events in the debug area.

In the http input node, select POST as a Method - as this is the default method in your voice application settings, and fill in the URL field with /event.

The http response node should have 200 set as Status code, but don't worry about it, this is the default value as well.

event webhookevent webhook

Serving Up the Audio File

On to the audio file you'd like to play into the call. If you have it somewhere online, make sure it is in either MP3 or WAV format, take note of the URL where it's hosted and continue with the next step.

Another option is serving it up from your computer. To do this, connect in sequence an http input node, a file in, a change and an http response node.

In the http input node, select GET as a Method and fill in the URL field with something like /filename.mp3, so that you can find your audio file at YOUR-URL/filename.mp3.

Next, open up the file in node properties, provide the absolute local path to the audio file you're serving up in the Filename field and set the Output to a single Buffer object. This will read the contents of the file as a binary buffer.

You also need to specify what type of file you're serving up, and that's where the change node comes into play. Set msg.headers to {} and msg.headers.content-type to audio/mp3.

serve up audio noderedserve up audio nodered

Now after hitting Deploy, point your browser at YOUR_URL/filename.mp3 and you should hear your audio file playing.

Creating a Nexmo Application

Some of Nexmo’s APIs, including the Voice API, use Nexmo Applications to hold security and config information needed to connect to Nexmo endpoints.

In the Nexmo Node-RED palette, several nodes have the ability to create these applications: getrecording, earmuff, mute, hangup, transfer, createcall, playaudio, playtts and playdtmf.

Let's use the createcall node, which will also be able to make an outbound call using the NCCO you'll build next.

Once you have it in your workspace, double-click on the createcall node to open up the node editor. Next to the Nexmo Credentials, select "Add new nexmovoiceapp..." from the drop-down menu and click the edit button. Fill in the details below and click Create New Application.

Name Choose a name for your Voice Application, for example Stream audio.
API Key Your Nexmo API key, shown in your account overview.
API Secret Your Nexmo API secret, shown in your account overview.
Answer URL The URL that Nexmo makes a request to when handling inbound calls. In case you're only interested in making an outbound call, just use - you won't be needing it. Otherwise, set it to YOUR_URL/answer, you'll be hosting a Nexmo Call Control Object (NCCO) here. - more about this later on.
Event URL Nexmo will send call events (e.g. ringing, answered) to this URL. If you’d like to receive events about the progress of your call, make sure your server is exposed to the internet, then use YOUR_URL/event for this field. Otherwise, feel free to use - this will respond with 200 OK. You could also override this eventURL for a specific createCall node in its node properties.

Node-RED will then create a new Nexmo Application on your account and fill in the App ID and Private Key fields for you to save. Now you can find this application in your Nexmo Dashboard under Voice > Your Applications.

create voice app examplecreate voice app example

Playing an audio file to a caller

Linking Your Virtual Number

Next you need to link your virtual number to the application created earlier by the createcall node.

Find the Voice Application you've just created in your Nexmo Dashboard by navigating to Voice > Your Applications. Click on the name of this application, then under the Numbers tab click on the Link button next to the virtual number you've rented earlier.

link number voicelink number voice

Building the Nexmo Call Control Object (NCCO)

Nexmo calls are controlled using Nexmo Call Control Objects, also known as NCCOs. An NCCO defines a list of actions to be followed when a call is handled. There are lots of different actions available, find the corresponding nodes under the Nexmo palette in your Node-RED editor or check out the NCCO Reference to find out more about them.

When handling inbound calls, you need your NCCO hosted at an Answer URL, and for this tutorial, you'll be using the stream action.

Drag and drop the stream node into your workspace, set the Stream URL, then connect it to a voice webhook input node and a return NCCO output node.

Next, in the voice webhook node, select GET as a method and type something like /answer in the answer URL field.

stream ncco answer urlstream ncco answer url

Finally, go to the createcall node properties, select URL from the Answer drop-down, and fill in the field with YOUR_URL/answer.

Call your linked Nexmo number, lay back and enjoy! Your audio file will be waiting for you. Psst, you can also follow your call events in the debug sidebar!

Streaming an Audio File into a Phone Call

Building the Nexmo Call Control Object (NCCO)

After building the NCCO, this will be passed on to the createcall node, which will then be used to make the outbound call. This createcall node takes 3 types of input for NCCO in the Answer field: JSON, URL or msg.ncco. Depending on which one you choose, there are 3 corresponding ways to build the NCCO, as follows.


Drag and drop the stream node into your workspace, double-click on it to open the node properties and set the Stream URL {} field to the link where your audio file is hosted -- YOUR_URL/filename.mp3. Note the {} sign next to the label, indicating that this value can be set dynamically, using Mustache templating. You can also set Barge In, Loopand Level values, although these are not required. See the Stream section of the NCCO reference to find out more.

Next, wire stream node's output into the createcall node, then under the createcall node properties select msg.ncco from the Answer drop-down menu.

stream msg nccostream msg ncco


If you'd rather write your NCCO as JSON, instead of using the action nodes, you can do so in the createcall node. Open the node properties and select JSON in the Answer field. Expand the JSON editor and paste in the snippet below:

        "action": "stream",
        "streamUrl": ["https://YOUR_URL/filename.mp3"]

Answer URL

Alternatively, you can serve up the NCCO at an AnswerURL. Drag and drop the stream node into your workspace, set the Stream URL, then connect it to a voice webhook input node and a return NCCO output node. Next, in the voice webhook node, select GET as a method and type /answer in the answer URL field.

stream ncco answer urlstream ncco answer url

Finally, go to the createcall node properties, select URL from the Answer drop-down, and fill in the field with YOUR_URL/answer.

Making the Outbound Call

Next, let's have a closer look at the createcall node properties. To actually make the outbound call, you need to fill in a few more details.

First, select Phone from the Endpoint drop-down menu. This will forward the call to a phone number, which you can specify in the text field next to the Number{} label.

Note the {} sign, which means that Mustache templating is supported for these fields. You could hardcode a phone number in here, or pass it in dynamically using an inject node, and in this case, referencing it with {{msg.payload}}.

Moving on to the next step, set one of your virtual numbers as the From{} number.

edit create calledit create call

Add an inject node to set off the flow and wire it as an input for the path containing the createcall node. In its node properties, you can select Number from the Payload drop-down menu, and fill in the text field next to it with the phone number you wish to call in E.164 format. For example 447401234567. In this case, don't forget to reference this number using {{msg.payload}} in the createcall node properties as the value of Number{}.

To have a bit more insight into what's happening when you make the call, wire the createcall's output into a debug node.

Now hit Deploy and click the inject node's button - your phone should be ringing any second now! You can also follow the flow of your call in the debug sidebar if you've implemented an event webhook.

stream outbound debugstream outbound debug

Where Next?

Julia BiroVonage Alumni

Julia is a Developer Advocate for Vonage focusing on low-code technologies and integrations. Maker and breaker of things, explorer of yet another builder tool. Forever challenging the barriers to entry of tech, she is passionate about learning and teaching, creating resources for the community.

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