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Warm Transfer with Java and Servlets

Have you ever been disconnected from a support call while being transferred to someone else?

Warm transfer eliminates this problem. Using Twilio powered warm transfers your agents will have the ability to dial in another agent in real-time.

Here is how it works at a high level:

  1. The first agent becomes available when he/she connects through the web client.
  2. The second agent also becomes available when he/she connects through the web client.
  3. A client calls our support line.
  4. The client stays on hold while the first agent joins the call.
  5. While the first agent is on the phone with the client, he/she can dial a second agent into the call.
  6. Once the second agent is on the call, the first one can disconnect from it. The client and the second agent stay on the call.

Let's get started! Clone the sample application from Github, and click the button below to begin.

We're Warming Up

Set Up the Voice Webhook

First let's configure the voice webhook for the Twilio number that customers will dial when they want to talk to a support agent.

Twilio Console for Warm Transfer

This should be the public-facing URL or your app in production: http://***


        Route for our Twilio webhook


        Awesome, now you've got a webhook in place. Next up, we'll look at some of the code.

        Connect an Agent

        Connecting an Agent

        Here you can see all front-end code necessary to connect an agent using Twilio's Voice Web Client.

        We essentially need three things to have a live web client:

        • A capability token (provided by our app)
        • A unique identifier (string) for each agent
        • Event listeners to handle different Twilio-triggered events

              Create a new call from the browser


              In the next step we'll take a closer look at capability token generation.

              Generate a Capability Token

              Generate a Capability Token

              In order to connect the Twilio Voice Web Client we need a capability token.

              To allow incoming connections through the web client an identifier must be provided when generating the token.


                    Generate a Capability Token


                    Next up let's see how to handle incoming calls.

                    Handle Incoming Calls

                    Handle Incoming Calls

                    For this tutorial we used fixed identifier strings like agent1 and agent2, but you can use any generated string for your call center clients. These identifiers will be used to create outbound calls to the specified agent using the Twilio REST API.

                    When a client makes a call to our Twilio number the application receives a POST request asking for instructions. We'll use TwiML to instruct the client to join a conference room and the Twilio REST API client to start a call with the first agent, so he can join the same conference.

                    When providing instructions to the client, we also provide a callback waitUrl, which in our case is another end point of our application. This returns more TwiML to say welcome to the user, and also play some music while on hold. Take a look at the code here

                    We use the client's CallSid as the conference identifier. Since all participants need this identifier to join the conference, we'll need to store it in a database so that we can grab it when we dial the second agent in later.


                          Create a conference call, connect the first agent, and put the client on hold


                          Now let's see how to provide TwiML instructions to the client.

                          Provide TwiML Instruction For The Client

                          Providing TwiML Instruction For The Client

                          Here we create a VoiceResponse using its Builder, and it will contain a Dial verb with a Conference noun that will instruct the client to join a specific conference room.


                                Generate TwiML for the client to join a conference call


                                Next up, we will look at how to dial our first agent into the call.

                                Bring in the First Agent

                                Dialing the First Agent Into the Call

                                For our app we created a callAgent method to handle dialing our agents. This method uses Twilio's Sdk Call class to create a new call. The create static method receives the following parameters:

                                1. To : The agent web client's identifier (agent1 or agent2)
                                2. From: Your Twilio number
                                3. Url : A URL to ask for TwiML instructions when the call connects

                                The create method returns an object which contains the execute method and the call to Twilio's API only happens when this execute method is called.

                                Once the agent answers the call in the web client, a request is made to the callback URL instructing this call to join the conference room where the client is already waiting.


                                      Dial an agent into the call


                                      With that in mind, let's see how to add the second agent to the call.

                                      Dial Second Agent Into the Call

                                      Dialing the Second Agent Into the Call

                                      When the client and the first agent are both in the call we are ready to perform a warm transfer to a second agent. The first agent makes a request and we look for his conferenceId needed to dial the second agent in. Since we already have a callAgent method we can simply call it to connect the second agent in.


                                            Invite a second agent to the conference


                                            Next up, we'll look at how to handle the first agent leaving the call.

                                            The First Agent Leaves the Call

                                            The First Agent Leaves the Call

                                            When the three participants have joined the same call, the first agent has served his purpose. Now he can drop the call, leaving agent two and the client to have a pleasant conversation.

                                            It is important to notice the differences between the TwiML each one of the participants received when joining the call. Both, agent one and two, have startConferenceOnEnter set to true. This means the conference will start when any of them joins the call. For the client calling and for agent two endConferenceOnExit is set to true. This causes the call to end when either of these two participants drops the call.


                                                  Instruction to dial into a conference call


                                                  That's it! We have just implemented warm transfers using Java Servlets. Now your clients won't get disconnect from support calls while they are being transferred to some else.

                                                  Where to next?

                                                  Where to next?

                                                  If you're a Java developer working with Twilio, you might also enjoy these tutorials:


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                                                  Thanks for checking this tutorial out! Let us know on Twitter what you've built... or what you're building.

                                                  David Prothero Kat King Samuel Mendes Brianna DelValle
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