Block Spam Calls and Robocalls with C#
Time to read: 4 minutes
January 10, 2017
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Spam, scams, and robocalls are at best annoying. For high-volume customer service centers, they can significantly impact the bottom line. Let’s leverage the Twilio Marketplace and our C# skills to block spam callers, robocallers, and scammers.
Before getting started, you should already know how to handle incoming calls with Twilio.
Get a Spam Filtering Add-on
You can integrate third-party technologies without leaving the comfort of the Twilio API. You can access the Add-ons from your Twilio Console. Today, we’re going to look at two Voice Add-ons that can help us with this spam problem: Marchex Clean Call, and Nomorobo Spam Score.
Installing the Add-on
Once you’ve decided on the Add-on you’d like to use, click the Install button and agree to the terms. In our use case today, we want to make use of these Add-ons while handling incoming voice calls, so make sure the Incoming Voice Call box for Use In is checked and click Save to save any changes:
Note the “Unique Name” setting. You need to use this in the code that you will write to read the Add-on’s results. In the code for this guide, we are sticking with the default names.
Check Phone Number Score in C#
When Twilio receives a phone call for your phone number, it will send details of the call to your webhook (more on how to configure that later). In your webhook code, you create a TwiML response to tell Twilio how to handle the call.
For spam-checking, our code needs to check the spam score of the number and deal with the call differently depending on whether the Add-on considers the caller to be a spammer or not. In our example code here, we’ll return a <Reject> TwiML tag to send spammers packing and a <Say> TwiML tag to welcome legit callers.
The code for our controller is part of a standard ASP.NET MVC project:
Notice the code has checks for the Add-ons we mentioned before. The code is written to be very flexible and handle missing data in the JSON response, so feel free to copy and paste even if you only plan to use one of the Add-ons.
How to Check Marchex Clean Call
Here’s an example of what Marchex Clean Call will return:
Working with that data in C# can be handled with:
``csharp
using System.Diagnostics;
using System.Web.Mvc;
using Newtonsoft.Json.Linq;
using Twilio.AspNet.Common;
using Twilio.AspNet.Mvc;
using Twilio.TwiML;
namespace BlockSpamCalls.Controllers
{
public class VoiceController : TwilioController
{
private const int WhitepagesBadReputation = 4;
private const int NomoroboSpamScore = 1;
private const string SuccessfulStatus = "successful";
[HttpPost]
public TwiMLResult Index(VoiceRequest request, string addOns)
{
var response = new VoiceResponse();
var isCallBlocked = false;
if (!string.IsNullOrWhiteSpace(addOns))
{
Trace.WriteLine(addOns);
var addOnData = JObject.Parse(addOns);
if (addOnData["status"]?.ToString() == "successful")
{
isCallBlocked = IsBlockedByNomorobo(addOnData["results"]?["nomorobo_spamscore"])
|| IsBlockedByWhitepages(addOnData["results"]?["whitepages_pro_phone_rep"])
|| IsBlockedByMarchex(addOnData["results"]?["marchex_cleancall"]);
}
}
if (isCallBlocked)
{
response.Reject();
}
else
{
response.Say("Welcome to the jungle.");
response.Hangup();
}
return TwiML(response);
}
private static bool IsBlockedByNomorobo(JToken nomorobo)
{
if (nomorobo?["status"]?.Value() != SuccessfulStatus) return false;
var score = nomorobo["result"]?["score"]?.Value();
return score == NomoroboSpamScore;
}
private static bool IsBlockedByWhitepages(JToken whitepages)
{
if (whitepages?["status"]?.Value() != SuccessfulStatus) return false;
var reputationLevel = whitepages["result"]?["reputation_level"].Value();
return reputationLevel == WhitepagesBadReputation;
}
private static bool IsBlockedByMarchex(JToken marchex)
{
if (marchex?["status"]?.Value() != SuccessfulStatus) return false;
var recommendation = marchex["result"]?["result"]?["recommendation"]?.Value();
return recommendation == "BLOCK";
}
}
}
```
How to Check Nomorobo Spam Score
Here’s an example of what Nomorobo Spam Score will return:
Finally, handling that data in C#:
Other Rejection Options
Using <Reject> is the simplest way to turn away spammers. However, you may want to handle them differently. The whole universe of TwiML is open to you. For example, you might want to record the call, have the recording transcribed using another Add-on, and log the transcription somewhere for someone to review.
Other Call Accept Options
For this example, we’re just greeting the caller. In a real-world scenario, you would likely want to connect the call using <Dial> (to call another number or Twilio Client), <Enqueue> the call to be handled by TaskRouter, or build an IVR using <Gather>.
Configure Phone Number Webhook
Now we need to configure our Twilio phone number to call our VoiceController whenever a call comes in. The URL for our controller is /voice, so we just need a public host for our ASP.NET application. You can publish your app to Azure or use ngrok to test locally.
Armed with the URL to the VoiceController, open the Twilio Console and find the phone number you want to use (or buy a new number). On the configuration page for the number, scroll down to "Voice" and next to "A CALL COMES IN," select "Webhook" and paste in the function URL. (Be sure "HTTP POST" is selected, as well.)
Testing a Blocked Call
You can quickly call your Twilio number to make sure your call goes through. However, how can we test a blocked spam result? The easiest way is to write some unit tests that pass some dummied up JSON to our controller action. For example, if we wanted to test a Nomorobo “BLOCK” recommendation, we could use the following JSON:
With that as our test fixture, we can write a test like the following to ensure that our call is blocked when we see the right data in the AddOns JSON:
For a full solution with all the tests for each of the Add-on providers, see our GitHub repository.
What’s Next?
As you can see, the Twilio Add-ons Marketplace gives you a lot of options for extending your Twilio apps. Next, you might want to dig into the Add-ons reference or perhaps glean some pearls from our other C# tutorials. Wherever you’re headed next, you can confidently put spammers in your rearview mirror.
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