Debugging Your Twilio Application

Integrating Twilio products into your software is easy, but as you are building and testing your application you may run into issues you did not anticipate. Here are some tips that we have found useful.

Break the Problem Down

The first thing to do is break the problem down into smaller parts. For instance, if you are using the REST API to make an outbound phone call, there are several things taking place:

  • Your code uses a helper library to invoke Twilio's REST API
  • The HTTP request is authenticated and parameters validated
  • An outbound phone call is placed
  • Once the call is answered an HTTP request made to the webhook you specified
  • The TwiML returned from the webhook is parsed and executed

Any one of those steps could experience an issue, whether it's a network issue or invalid TwiML being executed. If possible, test each of these steps independently to better isolate the issue.

Debugging Webhooks

If you're experiencing an issue with a webhook that you've configured, there are several things you can do to track down the source of the issue.

Check the Debugger

First, check out the Debugger. When Twilio runs into a problem with your webhook, it will log information about this in the Debugger. The Debugger flags these debugging events as either errors or warnings. Warnings mean that Twilio encountered an issue but was still able to process the request. An error is more severe and means that Twilio was not able to process the request at all. When you review these debugging events, you'll have access to:

  • The exact error or warning that occurred
  • Possible causes for this error
  • Suggested solutions
  • The entire HTTP request and response associated with this webhook request

Run the Webhook in Your Browser

Remember, you're just writing a web application. There's nothing Twilio does that you can't test right there in your browser. Visit the URLs in your web browser, and see that you don't have any errors.

  • Firefox treats XML files really nicely, highlighting any invalid XML in your document.
  • Mimic Twilio's data passing by manually adding data to your URLs. For example, if you ask Twilio to digits and the action is http://www.myapp.com/handleDigits.php, you can open your browser to http://www.myapp.com/handleDigits.php?Digits=1 to verify what happens if the user presses 1.
  • Make sure your application isn't sending debug output, because that will nearly always cause problems. You can, however, wrap any such output in XML comment blocks... they're the same as HTML comment blocks: <!-- COMMENTS HERE -->

Check for HTTP Redirects

Twilio is a well-behaved HTTP client, so when it receives an HTTP 301 or 302 redirect it will follow it to the specified URL. However, on the subsequent request the original parameters are not included. Occasionally you may see parameters such as Digits or RecordingUrl not arriving where you expect them. In this case, check to make sure the URL is not returning a redirect.

As an example, when a request is made to the action URL, a Digits parameter is included in the POST request. If the action URL redirects to another URL, Twilio will follow the redirect and issue a GET request to the specified URL. This GET request will include the standard set of parameters included with every Twilio request, but will not include the additional Digits parameter.

Common situations that may return unexpected redirects are:

  • A server that automatically redirects all HTTP requests to HTTPS
  • A URL rewriting rule that rewrites request URLs to include or exclude www.

To see what your server is returning to Twilio, create a test request using cURL, hurl.it or your HTTP client of choice and inspect the response returned from your URL.

Debugging Calls to the REST API

While errors and warnings related to Twilio invoking your webhooks are available in the the Debugger, the REST API will synchronously return an error object to your application in the event an error takes place. The error object will contain the HTTP response status code, Twilio-specific error code, error message, and a link to the Error Code Reference.

{
  "code": 21211,
  "message": "The 'To' number 5551234567 is not a valid phone number.",
  "more_info": "https://www.twilio.com/docs/errors/21211",
  "status": 400
}

Try Making the Call in the API Explorer

A great tool to use when debugging issues with the REST API is the Console's API Explorer. The API Explorer is a web-based tool that makes it easy to execute real calls against the REST API and examine the output. Use this tool to simulate the call that your application is making and compare its output to the output from your app.