NCCO Guide for Calling

In this guide, we'll demonstrate how to set up a dynamic answer URL with a NCCO that will allow you to configure more complex call flows from within your application via the Voice API. This includes the ability to make hybrid leg calls from your app (via WebRTC) to a phone (via PSTN) and vice versa. This guide will focus on making calls from your app to a PSTN phone. After completing this guide, you can follow along with the JavaScript, Android, or iOS guides.


This guide will introduce you to the following concepts:

Before you begin

To install the Nexmo CLI enter the following command:

$ npm install -g nexmo-cli@beta

To set up the CLI to use your API Key and API Secret use the following command:

$ nexmo setup api_key api_secret

Setup instructions

For this guide, you'll need to connect to an answer and event URL from either a preexisting Nexmo Application or a newly created one:

nexmo app:create "Stitch Outbound PSTN"


nexmo app:update aaaaaaaa-bbbb-cccc-dddd-0123456789ab "Stitch Outbound PSTN"

Then you'll need to buy a number with Nexmo and link the number to your new or existing Stitch application:

nexmo number:buy 16625461410
nexmo link:app 16625461410 aaaaaaaa-bbbb-cccc-dddd-0123456789ab

Using the SDKs with an answer URL

The new or existing application should be used to generate a user JWT that your user will use to login to one of the Stitch SDKs. For more info about creating a JWT and associating with a user or application. See the first In-App Messaging quickstart

When you use one of the Nexmo Stitch SDKs to make a PSTN call, you can pass in the phone number in E.164 format you want to call as an argument to that SDK's callPhone() method. However, you are not required to pass in only phone numbers. If you prefer, you can pass in any string such as a user ID or name. Then the Stitch API will make a request to your answer URL with the following parameters:


The to parameter will contain the string or phone number you passed into the SDK's callPhone method. This parameter can be used to dynamically tell Nexmo which phone number to call. If you choose to pass in a string other than a phone number, such as a user ID or name, you can use that reference to look up that user's phone number instead. The from_user parameter is the name of the Stitch user that initiated the call. The conversation_uuid is the ID of the conversation that is created for the IP and PSTN user for this call. The UUID parameter does not apply to this guide.

Setting up the answer and event URLs

If you'd like to set the caller's number the PSTN recipient will see when their phone rings, we can use the NCCO to set a from parameter with a Nexmo virtual number. This can be retrieved from what the SDK is forwarding (like the phone number, user Id or any other field that is passed) or other retrieved information(for example the result of a lookup for the phone number of a specific user Id).

Using the to parameter that Nexmo sends our answer URL and the NEXMO_NUMBER that you bought we can construct an NCCO like so:

  "action": "connect",
  "from": "NEXMO_NUMBER",
  "endpoint": [{
    "type": "phone",
    "number": "TO_NUMBER"

This is the NCCO that should be served from the answer URL, with NEXMO_NUMBER replaced with the number you've bought from Nexmo and TO_NUMBER replaced with the to query parameter that the Nexmo API serves your answer URL.

After you've implemented the callPhone() method and called it in your project using one of the Nexmo SDKs, the caller you just dialed should receive your call and see the from number that you've set with the NEXMO_NUMBER.

Making Calls as part of existing Conversations

NCCOs create new conversation objects by default. If you would like the interaction to be part of an already existing conversation, you need to, in addition to the connect action, to use the conversation action in the NCCO to provide the name of the conversation under which the call can be nested like so:

  "action": "conversation",

Where next?

An example of an answer URL with a dynamic NCCO object using the NEXMO_NUMBER and TO_NUMBER is available in the IP to PSTN glitch demo project

After reading this guide, you should read more about implementing outbound PSTN calling with the JavaScript, Android or iOS SDKs.

If you'd like to learn more about other actions you can use in your NCCO to control the flow of a call such as recording, sending audio files or synthesized speech, you can read more about that in the NCCO reference guide for the Voice API.