Forward a Call via Voice Proxy with ASP.NET Core
Published on May 13, 2021

This is the fifth tutorial on how to use Voice APIs with ASP.NET series.

In the previous tutorial, we learnt how to handle user input with ASP.NET Core.

Nowadays, we use a lot of apps and services that require us to communicate with another party, usually a stranger, via phone calls or messages. Think about food delivery or taxi booking apps.

One way to protect both parties is to mask their phone numbers by using an intermediary number. This is known as Voice Proxy.

Let's see how we can implement this technique using Vonage's Voice APIs.

Learning objectives

In this tutorial, we will:

  • Create an ASP.NET Core app.

  • Use NancyFX with ASP.NET Core.

  • Create a Vonage voice application.

  • Create and return NCCOs.

  • Run and test the code using Ngrok.


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.


To use The Vonage Voice API, we need to create a voice application.

The configuration steps are detailed in the Vonage Voice API with ASP.NET: Before you start post

Once the configuration is done successfully, we are ready to forward a call.

Forwarding a Phone Call

When a call is received, the Vonage Voice API will make a request to your application to figure out how to respond.

For that purpose, we are going to use NancyFX alongside our ASP.NET Core project.

First of all, we need to add Nancy to our project :

PM> Install-Package Nancy
PM> Install-Package Microsoft.AspNetCore.Owin

To allow Nancy to handle any HTTP requests, we need to tell ASP.NET Core to use Nancy via Owin in the Configure method of Startup.cs.

using Microsoft.AspNetCore.Builder;
using Nancy.Owin;

namespace NexmoVoiceASPNetCoreQuickStarts
    public class Startup
        public void Configure(IApplicationBuilder app)
            app.UseOwin(x => x.UseNancy());

The next step is to create a Nancy module in which we set up a route to /webhook/answer which will respond with the ncco returned by GetConnectNCCO()

using Nancy;
using Vonage.Voice.Nccos;
using Vonage.Voice.Nccos.Endpoints;

namespace NexmoVoiceASPNetCoreQuickStarts
    public class VoiceModule : NancyModule
        public VoiceModule()
            Get["/webhook/answer"] = x => { var response = Response.AsJson(GetConnectNCCO());
                                            response.ContentType = "application/json";
                                            return response;

        private Ncco GetConnectNCCO()
            var ncco = new Ncco();

            ncco.Actions.Add(new ConnectAction
                From = "VONAGE_NUMBER",
                Endpoint = new[] {
                    new PhoneEndpoint
                        Number = "TO_NUMBER"
            return ncco;

The above code will do the following:

When a call is received, Vonage will mask the original caller's number and instead use a virtual number as a facade to this phone call.

We are done! To test this sample app, some more configuration steps are required.

If you've been following so far, you've already configured your Vonage account and created a voice app as shown in this post. We need to link this app to a Vonage phone number that we are going to call. If you don't have a number, you can purchase one using the dashboard or the CLI:

vonage nubmers:search US
vonage numbers:buy [NUMBER] [COUNTRYCODE]

Similarly to link the number, you can use the dashboard or the CLI:

vonage apps:link --number=VONAGE_NUMBER APP_ID

We need to tell Vonage which URL to make a request to when a call is received - this is called the answer_url. For me, this url is http://localhost:63286/webhook/answer and that's only running locally. To expose our webhook answer url, we will use Ngrok.

ngrok http 63286

We now have a new url ( that can be used as the answer_url for the voice application. Update your application with your new answer_url. It should look like

Tada! Run the app and give it a go by calling the TO_NUMBER; you will notice that on the other side, you won't see the phone number you're using but instead your VONAGE_NUMBER.

Learn More

API References and Tools

Nexmo Getting Started Guides for ASP.NET

Rabeb OthmaniVonage Alumni

Rabeb was a Developer Advocate at Nexmo focusing on cloud communication APIs and helping developers get the best experience possible when building their apps. Other than writing code for a living, Rabeb advocates for bringing more women and minorities into tech, thus her involvement with Women Who Code and different tech communities. She leads the Women Who Code Network in Bristol.

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